Improve error reporting on users API

* users (#6878, #3526, #4209): Validation error messages are now exposed through 400 responses, 409 response are sent in case of duplicate email or username
* MRs (#5335): 409 responses are sent in case of duplicate merge request (source/target branches), 422 responses are sent when submiting MR fo/from unrelated forks
* issues
* labels
* projects