Raise nicer error from `show()` when missing a dummy data file

[evansd]

A present, the error you get from the show() command when you call it on an expression for which there is no corresponding dummy data file is a bit "computery":

Show line 25:
Error loading file 'analysis/dataset_definition.py':

Traceback (most recent call last):
  File "/workspace/analysis/dataset_definition.py", line 25, in 
    show(dataset.concordant_dates)
    ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
ehrql.file_formats.base.FileValidationError: Missing file: dummy_tables/repeat_medications_raw.csv

We could make this a bit more user-friendly.

Possibly we could generate the command they would need to run to create new dummy tables from their dataset.

This is related to:

• Behaviour of VSCode "erhQL debug" extension is very useful but also confusing #2674