Skip files of an unreadable blob under restore --skip-errors (closes #218)

A blob that failed to download ended `snapshot restore` even with
--skip-errors, after restoring whichever files came first. The download
error now goes through the same per-file handling as any other restore
error, once for every pending file that references the blob. With
--skip-errors those files are reported as failed, the rest are
restored, and the command still exits non-zero. Without the flag the
restore still aborts; the error now also names one affected file and
suggests --skip-errors. A cancelled restore still ends at once. The
--skip-errors help text and README now limit the packing and storage
caveat to snapshot creation.

Model: opus-5-5

internal/cli/root.go

@@ -60,7 +60,8 @@ on the source system.`,
 	cmd.PersistentFlags().BoolVar(&rootFlags.SkipErrors, "skip-errors", false,
 		"Skip files that cannot be read when creating a snapshot, or "+
 			"that cannot be restored when restoring, instead of aborting "+
-			"(packing and storage errors still abort)")
+			"(packing and storage errors while creating a snapshot still "+
+			"abort)")
 
 	// Add subcommands
 	cmd.AddCommand(