defer "can't load gundo" warning

Only inform that gundo didn't load when an attempt is made to invoke it.
Too many hosts still don't have vim 7.3, but I'd like to do a straight
update of my vimfiles from my dvcs without needing any fiddly
conditionals in order to load vim without a warning message.

plugin/gundo.vim

@@ -17,7 +17,10 @@ endif
 let loaded_gundo = 1"}}}
 
 if v:version < '703'"{{{
-    echo  "Gundo requires Vim 7.3+"
+	function! s:GundoDidNotLoad()
+		echohl WarningMsg|echomsg "Gundo unavailable: requires Vim 7.3+"|echohl None
+	endfunction
+	command! -nargs=0 GundoToggle call s:GundoDidNotLoad()
     finish
 endif"}}}