Currently, when a query can't be parsed, the error is:
- logged to Sentry (which is useless to us),
- returned as a generic 'Internal Server Error' (which is useless to the
user who made the query).
With this commit, the error is instead ignored from our logs (because it
is a user error), but the parse error details are returned to the user,
with the following format:
> {'errors': [{'message': 'Parse error on ")" (RPAREN) at [3, 23]'}]}
