tools: Print error and exit if invalid command is passed to idevicesyslog

tools/idevicesyslog.c

diff --git a/tools/idevicesyslog.c b/tools/idevicesyslog.c
index dc404b0..c8f4613 100644
--- a/tools/idevicesyslog.c
+++ b/tools/idevicesyslog.c
@@ -1224,6 +1224,9 @@
 				fclose(outf);
 			}
 			return 0;
+		} else {
+			fprintf(stderr, "Unknown command '%s'. See --help for valid commands.\n", argv[0]);
+			return 1;
 		}
 	}