Add utility functions for classifying errors

[mdbooth]

TODO:

• [ ] Unit tests

[pierreprinetti]

Here is a proposal to make sentinel errors useful in the core library: https://github.com/gophercloud/gophercloud/pull/2904 Can you please take a look and see if it covers your case?

[pierreprinetti]

Tentatively closing. @mdbooth please reopen if the proposed solution is not covering your use-case.