Requests can accumulate errors with no obvious indication, e.g. if
their primary purpose is to construct a URL: URL() itself doesn't
return an error if r.err is non-nil.
Instead of changing URL() to return an error, which has quite a large
impact, add an Error() function and indicate on URL() that it should
be checked.
Signed-off-by: Stephen Kitt <skitt@redhat.com>
Kubernetes-commit: f69c1c47463ff70ad61adf6f38c4d5b7373e9d0a
22e2a9b6b1