Better error messages for `case()`, `when()`, `then()` misuse

[evansd]

This doesn't add any new invalid constructs (apart from the edge case of duplicated conditions) but it does raise what are hopefully much more helpful errors for already invalid constructs.

For example, compare the before and after errors for missing .otherwise():

Expecting an ehrQL series, got type 'WhenThen'

---

Missing `.otherwise(...)` condition on a `when(...).then(...)` expression

For using the otherwise() method where you should use the keyword argument:

AttributeError: 'StrPatientSeries' object has no attribute '_condition'

---

TypeError: invalid syntax for `otherwise` in `case()` expression, instead of:

    case(
        when(...).then(...).otherwise(...)
    )

You should write:

    case(
        when(...).then(...),
        otherwise=...
    )

And for not providing a non-null value:

AssertionError: Could not match TypeVar ~T in Case(cases={Function.EQ(lhs=SelectColumn(source=
SelectPatientTable(name='patients', schema=TableSchema(date_of_birth=Column(datetime.date, constraints=()), 
i=Column(int, constraints=()), f=Column(float, constraints=()))), name='i'), rhs=Value(value=0)): None}, 
default=None)

---

TypeError: `case()` expression cannot have all `None` values

Closes #1440

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