improved error-messages for body-validation

[MikeKrueger75]

continues #82

Is your feature request related to a problem? Please describe.
If anrede is present with null-value, the response-code 400 is given with message "Request Body ist leer oder nicht lesbar."

Describe the solution you'd like
A more descriptive message like "if a field expects an ENUM, a value must be specified (Anrede)}" is more helpful to solve the problem on client-side.