mdk install: Using -e without -i should throw an error

[danpoltawski]

It makes sense if you know that the only way mdk tracks external state is with config.php, but isn't necessarily obvious.

And what I mistakenly did was to create, then install, then run phpunit tests without realising I was using the wrong engine. (In fact I only noticed when I was looking for mysql hammering me cpu)

[FMCorz]

Reopening because of 9864334.