send usage to stderr on error paths

pbc

@@ -221,7 +221,7 @@ while (( $# > 0 )); do
       -c|--config)
         cfgfile=$1
         shift
-        [ -z "$cfgfile" ] && usage && exit 255
+        [ -z "$cfgfile" ] && echo "Error: missing argument for $opt" >&2 && usage >&2 && exit 255
       ;;
       -h|--help)
         usage
@@ -232,8 +232,8 @@ while (( $# > 0 )); do
       ;;
       -*)
         echo "Error: invalid option: $opt" >&2
-        echo
+        echo >&2
-        usage
+        usage >&2
         exit 255
       ;;
       *)
@@ -247,14 +247,14 @@ while (( $# > 0 )); do
       ;;
       -*)
         echo "Error: invalid option: $opt" >&2
-        echo
+        echo >&2
-        usage
+        usage >&2
         exit 255
       ;;
       *)
         echo "Error: invalid argument: $opt" >&2
-        echo
+        echo >&2
-        usage
+        usage >&2
         exit 255
       ;;
     esac