Redirects should not be considered errors

[mattdowdell]

At the moment, when an API returns a redirect, an exception is raised. This is contrary to the spec, i.e. RFC 7231, which considers 4XX and 5XX to be errors, but not 3XX. As such, the exceptions raised when such statuses are returned should be removed to be compliant with the spec.

Follows on from #449 and #454 where it was pointed out that this would be a breaking change.