Comment 0 for bug 1761475

I suggest adding optional severity to each error in http://specs.openstack.org/openstack/api-wg/guidelines/errors.html. This idea came from reviewing https://review.openstack.org/#/c/494534/ which is not aligned with our guidelines and does have severity as part of the error code. One use case for severity is to decide which of the errors in the errors list is the most critical. Another is for UI to display errors with different colors.

I cannot decide if we should use numeric constants or strings. Numeric constants can be sorted, strings are more user-friendly.