Fail restore for a requested path that matches nothing (closes #309)

snapshot restore said nothing about a requested path that matched no
entry in the snapshot (a typo, a relative path, or a path through a
configured snapshot path that is a symlink, whose entries are stored
under the resolved directory) and exited 0, also when nothing matched
at all. Each such path is now named in a warning, and restore returns
an error after restoring the paths that do match.

Making a path through a symlink match is not part of this change.
Judgement call: when files also fail to restore, that error is the one
returned; the unmatched paths are still named.

Model: opus-5-5

README.md

@@ -370,7 +370,9 @@ on the destination in one go, use `vaultik remote nuke --force`.
   store — its remote key (or an unambiguous leading part of it) as shown
   by `snapshot list`. See
   [restoring on another machine](#restoring-on-another-machine).
-* Optional path arguments to restore specific files/directories (default: all)
+* Optional path arguments to restore specific files/directories (default: all).
+  A path that matches nothing in the snapshot is named in a warning, and the
+  command exits non-zero after restoring the paths that match
 * Preserves file permissions, timestamps, ownership (ownership requires root),
   symlinks, and empty directories
 * `--verify`: After restoring, verify every file's chunk hashes match