Previoiusly, we would just bomb out with a KeyError("Foo") if a user
configured a repository Type=Foo. Or in case he tried to sync from a
Maildir to a Maildir. Still abort with an Exception now, but with one
that explains what actually had happened.
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>
