Output error message when a command is not found. Closes #778

Plugins with commands will need to implement a catch-all command that exits with the `DOKKU_NOT_IMPLEMENTED_EXIT`` code (10). This signals to dokku that a given plugin has indeed not executed anything for a plugin (which may not always be the case).

Using plugins that do not implement this pattern will result in those plugins silencing the error message.

plugins/git/commands

@@ -58,4 +58,8 @@ EOF
     cat
     ;;
 
+  *)
+    exit $DOKKU_NOT_IMPLEMENTED_EXIT
+    ;;
+
 esac