Should warn when backup destination doesn't exist

[Milan Bouchet-Valat]

If you forget to add a third / to the backup path, like file://mnt/somedir, duplicity doesn't warn, and later fails to find previous signatures. Same if you happen to pass a non-existent directory by mistake.

Instead of saying:
Fatal Error: Unable to start incremental backup. Old signatures not found and incremental specified
and instead of starting a full backup when incremental was expected, it would be nicer to precise that backup directory doesn't exist. It's much clearer for users.

For the specific case of file:// lacking a /, it would be even better to give the user a hint that the URI was likely mistyped if the directory turns out not to exist.

(This is with duplicity 0.6.10-0ubuntu1.)