Add option to add clarifying message

After usage message, before exiting---that's the sweet spot.

src/include-bash/pretty.sh

     #
     # Echo out usage patterns and (by default) `exit 2`
     #
-    #     mkusage [-e STATUS] [-E] pattern [patern...]
+    #     mkusage [-w MSG] [-e STATUS] [-E] pattern [patern...]
     #
     # Each pattern is prefixed by "usage: " and a resolved
     # script/function name to produce traditional usage hint.
     # that would be fed to `exit` will be used as exit status
     # of this function.
     #
+    # Optionally, you can add -w MSG to add clarifying message
+    # (presented as warning) to help user understand what is
+    # wrong with arguments they have passed.
+    #
     # Use "--" to delimit end of arguments processed by mkusage.
     #
     # Recommended usage is to define usage() in your script and
     # use this in its body.  That way you only need to define
     # usage patterns once and skip to them from any place where
-    # you detect incorrect usage.
+    # you detect incorrect usage.  Optionally, this usage()
+    # function can pass first argument as -w for clarification
+    # messages.
     #
     local es=$EXIT_USAGE    # our exit status
     local doexit=true       # should we exit?
+    local cmsg              # clarification message
     while true; do case "$1" in
         -e) es="$2";        shift 2 || return 2 ;;
         -E) doexit=false;   shift ;;
         -k) es=$EXIT_OK;    shift ;;
+        -w) cmsg="$2";      shift 2 || return 2 ;;
         --)                 shift; break ;;
         *)                  break ;;
     esac done
     _pretty__echo -u "$@";
+    test -n "$cmsg" && warn "bad usage: $cmsg"
     $doexit && exit "$es"
     return "$es"
 }