Return a descriptive error if a payload contains an untranslatable character

**Motivation**
PostgreSQL does not accept the null character (`u0000`) inside a JSON text field. A client uploading a JSON containing such text field will get a internal sever error response. However we should consider it as a client error and state what the issue is.

**Acceptance criteria**
1. When a user `POST`s or `PUT`s a payload that contains the `u0000` untranslatable character, an error from the 4XX class should be returned, specifying the problem.