bad_request should also return an error code

[jerryfletcher21]

Describe the bug
Every requests made that fails is returned a json response with a string field bad_request specifying the error.
For a client that needs to handle the failed requests it is not comfortable working with these strings.
It also can happen in the backend to change the string of a particular error, and if the client is checking for the previous string it will not catch it.

Expected behavior
Every bad_request should also include an int like error_code, for example like this:

{
    "error_code": 10,
    "bad_request": "The PGP signed cleartext message is not valid."
}