Provide better error message for expected column key error in assurance testing

[alarthast]

Reported on Slack by a user in this thread.

If a key in the user's provided expected_columns does not appear in the actual dataset, Python raises a KeyError with an error message that might not be immediately helpful to the user:

File "/app/ehrql/assurance.py", line 71, in validate
         if (validation_result := validate_patient(patient_id, patient, query_results))
                                  ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       File "/app/ehrql/assurance.py", line 139, in validate_patient
         if expected[k] != actual[k]
                           ~~~~~~^^^
     KeyError: 'reps_ids_differ'

We might want to catch the mismatch earlier and return a custom error message.