Some CSI plugins don't return much for errors over the gRPC socket
above and beyond the bare minimum error codes. This changeset improves
the operator experience by unpacking the error codes when available
and wrapping the error with some user-friendly direction.

Improving these errors also revealed a bad comparison with
`require.Error` when `require.EqualError` should be used in the test
code for plugin errors. This defect in turn was hiding a bug in volume
validation where we're being overly permissive in allowing mount
flags, which is now fixed.