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

Closes #975 (closed)

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