Expose error type checker for rest client

[rikatz]

Is your feature request related to a problem? Please describe.
There is no way today to check for an error type of rest Client. Because it is a private struct (https://github.com/vmware/govmomi/blob/v0.33.1/vapi/rest/client.go#L150) we cannot check if the returned error was not found, or something else.

At least creating an util that checks if an error is statusError and if it matches the desired status code would be really helpful

Describe the solution you'd like
Expose an util function for checking the returned error code from statusError

Describe alternatives you've considered
Right now the only alternative is to check the error string

Additional context
Add any other context or screenshots about the feature request here.