plugin/ruby: add missing parameter error message

plugins/available/ruby.plugin.bash

@@ -13,5 +13,5 @@ function remove_gem() {
 	param '1: installed gem name'
 	group 'ruby'
 
-	gem list | grep "${1?}" | awk '{ print $1; }' | xargs sudo gem uninstall
+	gem list | grep "${1:?${FUNCNAME[0]}: no gem name provided}" | awk '{ print $1; }' | xargs sudo gem uninstall
 }