* Give descriptive error if auth method not found
Previously during a `consul login -method=blah`, if the auth method was not found, the
error returned would be "ACL not found". This is potentially confusing
because there may be many different ACLs involved in a login: the ACL of
the Consul client, perhaps the binding rule or the auth method.
Now the error will be "auth method blah not found", which is much easier
to debug.
