Return 400 errors on deserialization failure

[rkorsak]

Feature Request

Until we have full Pydantic-compliant models (#98), we rely on custom deserialization in the routes, e.g.:

ballots = [PlaintextBallot.from_json_object(ballot) for ballot in request.ballots]

If this deserialization fails, however, it manifests as an opaque 500 error to the client. We should capture deserialization errors and send a 400 with as much information as possible.

[gagandeepp]

Interested please assgn

[keithrfung]

@gagandeepp you got it!

[gagandeepp]

@rkorsak how to reproduce this error?
Also do we need to make changes for all the scenarios in this ticket?

[keithrfung]

@gagandeepp This error can be replicated by passing any model that doesn't match the request model. Using the example:
ballots = [PlaintextBallot.from_json_object(ballot) for ballot in request.ballots]
will fail if there is an issue with ballot object such as a missing field, a string passed instead of a number, etc.

Yes, it should be addressed at all sections where a model is parsed. Probably a wrapping method.