[relDB] Improve checker

• Currently most of the values types are verified with the helper methods defined in cheker.py. The place where these methods are called is marked by the first TODO in the file. Desired changes:
  • Definevalidate()methods for all the field types
  • Remove check around the TODO
  • Catch type error from the field.validate() method inside of the check_normal_fields() method and assigned more clear messages (the ones that are currently used in the check after the first TODO)
• Check code, unreachable because of the validations duplication. Example: second TODO
• Check spelling in the error messages

Start in cheker.py, locating the first and second TODOs and the existing helper methods used for field-type verification. Trace the calls into check_normal_fields() and review the current error messages. Done means all field types define validate() methods, duplicated checks and unreachable code are removed, type errors receive clear messages, and spelling is corrected.