* 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.
Luke Kysow