return exit code 10 if repository does not exist

cmd/restic/cmd_stats.go

@@ -51,6 +51,7 @@ EXIT STATUS
 
 Exit status is 0 if the command was successful.
 Exit status is 1 if there was any error.
+Exit status is 10 if the repository does not exist.
 Exit status is 11 if the repository is already locked.
 `,
 	DisableAutoGenTag: true,