This removes the requirement on `restic self-update --output` to point
to a path of an existing file, to overwrite. In case the specified
path does exist we still want to verify that it's a regular file,
rather than a directory or a device, which gets overwritten.
We also want to verify that a path to a new file exists within an
existing directory. The alternative being running into that issue
after the actual download, etc has completed.
While at it I also replace `errors.Errorf` with the more appropriately
verbose `errors.Fatalf`.
Resolves #2491
