Better error handling for faulty OP

[peppelinux]

Let's assume that a OP have an unhandled exception that returns to RP a stupid HTML page (error 40x, 50x). We should take this into account

oidcrp/util.py", line 555, in load_registration_response
    raise OidcServiceError(response.to_json())
AttributeError: 'dict' object has no attribute 'to_json'