Error Handling - Better validation on PUT and PATCH operations

[dzahariev]

When I try to register broker, or platform, I may get a 500 as response without clear reason what is wrong in my payload.
APIs need to have better validation on PUT, PATCH and POST requests and to return back the message with meaningful information what was wrong in payload.

Example of such messages are:
"Wrong broker URL"
"Wrong broker credentials"
"Malformed broker URL"
"Invalid payload format (Not a valid JSON)"
"Missing label for plan <>"
"Too long broker name - allowed size is XX"