Bugfix: Make `mount` not create missing mount point directory

When specifying a non-existent directory as mount point for the `mount`
command, restic used to create the specified directory automatically.

This has now changed such that restic instead gives an error when the
specified directory for the mount point does not exist.

https://github.com/restic/restic/issues/1681
https://github.com/restic/restic/pull/3008