Exit non-zero when snapshot create --skip-errors skips anything (closes #310)

A backup under --skip-errors printed its skip errors and failed count,
then "Finished successfully.", and exited 0, so a cron wrapper could
not tell it from a complete backup. A directory the walk could not list
was not counted at all.

Each entry the walk skips is now counted as examined and failed. When
anything failed under --skip-errors, the run keeps its snapshots, runs
the post-backup prune as before, and then returns an error that the
command reports as "Backup completed, but N file(s) failed to back up".

Judgement call: without --skip-errors a file removed during the run is
still counted as failed and the exit status stays 0.

Model: opus-5-5

internal/cli/snapshot.go

@@ -115,6 +115,13 @@ on Linux), then /etc/vaultik/config.yml.`,
 					return
 				}
 
+				if errors.Is(err, vaultik.ErrFilesFailedBackup) {
+					log.Error("Backup completed with failed files", "error", err)
+					ReportErrorf("Backup completed, but %v", err)
+
+					return
+				}
+
 				log.Error("Snapshot creation failed", "error", err)
 				ReportErrorf("Snapshot creation failed: %v", err)
 			})