error, don't warn about insecure creds

A one-time warning was not good enough. A hard error will force the user to
notice the problem.

Perhaps worth noting that git-annex enableremote already failed with an
error, and nobody reported a bug. Suggests that not many people have used
the insecure configuration, or if they did, they went to the bother to
embedcreds, but never re-enabled the special remote.