Make GraphQL user warnings/errors silent

[mcg-web]

Q	A
Bug report?	yes
Feature request?	no
BC Break report?	no
RFC?	no
Version/Branch	master

see webonyx/graphql-php#553

[glye]

This seems needed. I see no way currently to prevent it from showing exceptions with stack traces on query parse exceptions. There should be a way to ensure all exceptions are caught, and a more generic error message is returned instead.

See also #1121