Refactor error response

[dougthor42]

This refactors how error responses are done in the flask routes.

Previously there was a a lot of duplicated code and it was also somewhat difficult to read. This replaces those instances with a single-line class method.

Before

http_status = 404
detail = "No data exists for metric '{}'.".format(metric)
resp = utils.Rfc7807ErrorResponse(
    type_="no-data",
    title="No data for metric",
    status=http_status,
    detail=detail,
)
logger.warning("API error: %s" % detail)
return resp.as_response(), http_status

After

return ErrorResponse.metric_has_no_data(metric)