fatal message causes rc=2 so replace with error

This allows the usage information to be shown when a bad command-line
option is used, and rc=3 to be returned.

src/backupninja.in

@@ -468,8 +468,7 @@ while [ $# -ge 1 ]; do
          ;;
       *)
          debug=1
-         echo "Unknown option $1"
-         fatal "Unknown option $1"
+         error "Unknown option $1"
          usage
          exit 3
          ;;