Standardise error response structure

[ialarmedalien]

There are a variety of different formats for error messages:

JSON decode error:

    resp = {
        'error': 'Unable to parse JSON',
        'source_json': err.doc,
        'pos': err.pos,
        'lineno': err.lineno,
        'colno': err.colno
    }

403 / Unauth error:

    resp = {
        'error': {
            'status': 403,
            'message': 'Unauthorized',
            'auth_url': err.auth_url,
            'auth_response': err.response,
        },
    }

404 / Not found:

    resp = {
        'error': {
            'message': 'Not found',
            'status': 404,
        }
    }

Generic 500:

    resp = {'error': {'status': 500, 'message': 'Unexpected server error'}}
    # TODO only set below two fields in dev mode
    resp['error']['class'] = err.__class__.__name__
    resp['error']['details'] = str(err)

Decide on a standard format for error messages and implement it uniformly across the different error types.