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

Restic used to exit with code 0 even when some backup sources did not exist. Restic
would exit with code 3 only when child directories or files did not exist. This
could cause confusion and unexpected behavior in scripts that relied on the exit
code to determine if the backup was successful.

Restic now exits with code 3 when some backup sources do not exist.

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