Resolve "/api/v1/token-auth does not return json on login failure"

Simcha van Collem
2020-02-20T08:45:53+01:00

Closes #975

#### Previous behaviour
Steps to reproduce:

1. Post wrong credentials to `/api/v1/token-auth`
2. "Unauthorized" is returned

#### New behaviour
Steps to validate that it works:

1. Post wrong credentials to `/api/v1/token-auth`
2. `{"error": "Unauthorized"}` is returned