Bugfix: Exit with code 3 when some `backup` source paths do not exist

Restic used to exit with code 0 when a top-level backup source path was
missing, and exited with code 3 only when a child path under an existing
source did not exist. Scripts that relied on the exit code could therefore
treat an incomplete backup as success.

Restic now exits with code 3 when any backup source path does not exist.

https://github.com/restic/restic/issues/4467
https://github.com/restic/restic/pull/5347
