Catch exception about missing / invalid credentails

... and provide a useful warning-level message about how to fix it.
I agree that this feels weird, but, in the case of an out-of-date password, you won't know the creds are bad until you try to actually make the POST.

I feel like this is the equivalent to something like ActiveRecord::ConnectionNotEstablished. We're coupling directly to the ActiveModel interface here, so we don't really have a great opportunity to abstract away any problems.