Lazy validation raises misleading SchemaError when columns are missing

Describe the bug A clear and concise description of what the bug is.

[x] I have checked that this issue has not already been reported.
[x] I have confirmed this bug exists on the latest version of pandera.
[ ] (optional) I have confirmed this bug exists on the main branch of pandera.

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Code Sample, a copy-pastable example

import polars as pl
import pandera.polars as pa

schema = pa.DataFrameSchema(
    {
        "a": pa.Column(pl.Int32),
        "b": pa.Column(pl.Int32)
    }
)

df = pl.DataFrame({"a": [1, 2, 3]})

schema.validate(df, lazy=True)
>> SchemaError: type String is incompatible with expected type Null

Expected behavior

Expected SchemaErrors (rather than SchemaError) which details the missing column and any other validation failures in the DataFrame.

Desktop (please complete the following information):

OS: macOS Sonoma Version 14.5
Browser: Chrome
Version: 0.19.3
Python Version: 3.11

Screenshots

If applicable, add screenshots to help explain your problem.

Additional context

It looks like the SchemaErrors class is attempting to concatenate DataFrames representing different failure cases into one DataFrame. In this case, the below are the DataFrames it is trying to concat:

shape: (1, 6)
┌──────────────┬─────────────────┬────────┬────────────────────┬──────────────┬───────┐
│ failure_case ┆ schema_context  ┆ column ┆ check              ┆ check_number ┆ index │
│ ---          ┆ ---             ┆ ---    ┆ ---                ┆ ---          ┆ ---   │
│ str          ┆ str             ┆ null   ┆ str                ┆ i32          ┆ i32   │
╞══════════════╪═════════════════╪════════╪════════════════════╪══════════════╪═══════╡
│ b            ┆ DataFrameSchema ┆ null   ┆ column_in_datafram ┆ null         ┆ null  │
│              ┆                 ┆        ┆ e                  ┆              ┆       │
└──────────────┴─────────────────┴────────┴────────────────────┴──────────────┴───────┘
shape: (1, 6)
┌──────────────┬────────────────┬────────┬────────────────┬──────────────┬───────┐
│ failure_case ┆ schema_context ┆ column ┆ check          ┆ check_number ┆ index │
│ ---          ┆ ---            ┆ ---    ┆ ---            ┆ ---          ┆ ---   │
│ str          ┆ str            ┆ str    ┆ str            ┆ i32          ┆ i32   │
╞══════════════╪════════════════╪════════╪════════════════╪══════════════╪═══════╡
│ Int64        ┆ Column         ┆ a      ┆ dtype('Int32') ┆ null         ┆ null  │
└──────────────┴────────────────┴────────┴────────────────┴──────────────┴───────┘

It should be as simple as setting the dtype of column to always be str but I have no knowledge of the pandera codebase, so no idea if it's actually as quick of a fix as I think it is 😂

Full Traceback:

SchemaError                               Traceback (most recent call last)
Cell In[6], line 1
----> 1 schema.validate(df, lazy=True)

File ~/.local/share/virtualenvs/zeus-XZJKObwC/lib/python3.11/site-packages/pandera/api/polars/container.py:58, in DataFrameSchema.validate(self, check_obj, head, tail, sample, random_state, lazy, inplace)
     54     if is_dataframe:
     55         # if validating a polars DataFrame, use the global config setting
     56         check_obj = check_obj.lazy()
---> 58     output = self.get_backend(check_obj).validate(
     59         check_obj=check_obj,
     60         schema=self,
     61         head=head,
     62         tail=tail,
     63         sample=sample,
     64         random_state=random_state,
     65         lazy=lazy,
     66         inplace=inplace,
     67     )
     69 if is_dataframe:
     70     output = output.collect()

File ~/.local/share/virtualenvs/zeus-XZJKObwC/lib/python3.11/site-packages/pandera/backends/polars/container.py:122, in DataFrameSchemaBackend.validate(self, check_obj, schema, head, tail, sample, random_state, lazy, inplace)
    120         check_obj = self.drop_invalid_rows(check_obj, error_handler)
    121     else:
--> 122         raise SchemaErrors(
    123             schema=schema,
    124             schema_errors=error_handler.schema_errors,
    125             data=check_obj,
    126         )
    128 return check_obj

File ~/.local/share/virtualenvs/zeus-XZJKObwC/lib/python3.11/site-packages/pandera/errors.py:183, in SchemaErrors.__init__(self, schema, schema_errors, data)
    178 self.schema_errors = schema_errors
    179 self.data = data
    181 failure_cases_metadata = schema.get_backend(
    182     data
--> 183 ).failure_cases_metadata(schema.name, schema_errors)
    184 self.error_counts = failure_cases_metadata.error_counts
    185 self.failure_cases = failure_cases_metadata.failure_cases

File ~/.local/share/virtualenvs/zeus-XZJKObwC/lib/python3.11/site-packages/pandera/backends/polars/base.py:204, in PolarsSchemaBackend.failure_cases_metadata(self, schema_name, schema_errors)
    198         failure_cases_df = pl.DataFrame(scalar_failure_cases).cast(
    199             {"check_number": pl.Int32, "index": pl.Int32}
    200         )
    202     failure_case_collection.append(failure_cases_df)
--> 204 failure_cases = pl.concat(failure_case_collection)
    206 error_handler = ErrorHandler()
    207 error_handler.collect_errors(schema_errors)

File ~/.local/share/virtualenvs/zeus-XZJKObwC/lib/python3.11/site-packages/polars/functions/eager.py:187, in concat(items, how, rechunk, parallel)
    184     out = wrap_df(plr.concat_df(elems))
    185 elif how == "vertical_relaxed":
    186     out = wrap_ldf(
--> 187         plr.concat_lf(
    188             [df.lazy() for df in elems],
    189             rechunk=rechunk,
    190             parallel=parallel,
    191             to_supertypes=True,
    192         )
    193     ).collect(no_optimization=True)
    195 elif how == "diagonal":
    196     out = wrap_df(plr.concat_df_diagonal(elems))

SchemaError: type String is incompatible with expected type Null

unionai-oss / pandera