Status 500 if edit call violates foreign key constraint

[AlexanderYAPPO]

If one tries to edit a language object and new values are not presented in the DB then status 500 will be returned.

It happens because we don't check if these new values already exist in the DB. If they're not here then we violate foreign key constraint and an exception is raised:

sqlalchemy.exc.IntegrityError: (psycopg2.IntegrityError) insert or update on table "dictionaryperspective" violates foreign key constraint "dictionaryperspective_parent_object_id_fkey"

We need to add try/catch blocks and return a proper answer.
Example of a problem code:

if language:
            if not language.marked_for_deletion:
                # TODO: Status 500 will be returned if arguments are invalid; add try/catch
                req = request.json_body
                if 'parent_client_id' in req:
                    language.parent_client_id = req['parent_client_id']
                if 'parent_object_id' in req:
                    language.parent_object_id = req['parent_object_id']
                if 'translation_gist_client_id' in req:
                    language.translation_gist_client_id = req['translation_gist_client_id']
                if 'translation_gist_object_id' in req:
                    language.translation_gist_object_id = req['translation_gist_object_id']
                request.response.status = HTTPOk.code
                return response