HTTP code should be set to 400 in case of improper payload sent to the user-feedback endpoint

[tisnik]

Currently, the user-feedback endpoint does not contain check if the payload sent to this endpoint is correct according to (any) schema.

Possible fix:

1. check payload right after it is received
2. respond accordingly

Report made by BAF can be seen here: https://fabric8-analytics.github.io/fuzz-tests/server_user_feedback_issue_450.htm