Fix EZP-20850: Creating an image via REST fails

This PR fixes issue https://jira.ez.no/browse/EZP-20850

Reported issue exposed that field languages in create and update structs were not validated.

Validation is added and excpetion messages are amended with language code information.
Additionally ```ContentValidationException``` and ```ContentFieldValidationException``` are now handled in REST server with HTTP 400 Bad Request.