Improved error handling to gracefully manage invalid inputs

[vasantvohra]

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[ThiefMaster]

FWIW this is the error I get when sending {"field_825794":{"lol":69}} as the payload to update a registration:

  File "/home/adrian/dev/indico/py3/src/indico/modules/events/registration/util.py", line 487, in modify_registration
    new_data = snapshot_registration_data(registration)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/adrian/dev/indico/py3/src/indico/modules/events/registration/util.py", line 951, in snapshot_registration_data
    'friendly_data': regdata.friendly_data}
                     ^^^^^^^^^^^^^^^^^^^^^
  File "/home/adrian/dev/indico/py3/src/indico/modules/events/registration/models/registrations.py", line 829, in friendly_data
    return self.get_friendly_data()
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/adrian/dev/indico/py3/src/indico/modules/events/registration/models/registrations.py", line 836, in get_friendly_data
    return self.field_data.field.get_friendly_data(self, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/adrian/dev/indico/py3/src/indico/modules/events/registration/models/form_fields.py", line 100, in get_friendly_data
    return self.field_impl.get_friendly_data(registration_data, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/adrian/dev/indico/py3/src/indico/modules/events/registration/fields/choices.py", line 237, in get_friendly_data
    caption = registration_data.field_data.field.data['captions'][uuid]
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
KeyError: 'lol'

So there are indeed cases that need to be fixed. In fact, we should probably fail validation earlier when there's data that's not a valid choice (careful so it still works in case of a multiselect field where one of the already-selected choices has been removed, since you can still have and keep it in an existing registration).