Raise exception when path points to non-existent file.

[drewoldag]

Problem & Solution Description (including issue #)

Add an exception when a path value is provided to set_data and it does not point to a file.

Code Quality

• [ ] My code follows the code style of this project
• [ ] I have written unit tests or justified all instances of #pragma: no cover; in the case of a bugfix, a new test that breaks as a result of the bug has been added
• [ ] My code contains relevant comments and necessary documentation for future maintainers; the change is reflected in applicable demos/tutorials (with output cleared!) and added/updated docstrings use the NumPy docstring format
• [ ] Any breaking changes, described above, are accompanied by backwards compatibility and deprecation warnings

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (4e93655) 96.95% compared to head (a3f5c16) 96.96%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #68 +/- ## ======================================= Coverage 96.95% 96.96% ======================================= Files 32 32 Lines 1709 1711 +2 ======================================= + Hits 1657 1659 +2 Misses 52 52 ``` | [Files](https://app.codecov.io/gh/LSSTDESC/rail_base/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC) | Coverage Δ | | |---|---|---| | [src/rail/core/stage.py](https://app.codecov.io/gh/LSSTDESC/rail_base/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC#diff-c3JjL3JhaWwvY29yZS9zdGFnZS5weQ==) | `99.15% <100.00%> (+0.01%)` | :arrow_up: |

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