Don't pick a random code if more than one matching are found

[giovannipizzi]

A user in the tutorial installed everything and also imported the Archive from the Materials Cloud.
Then, he set up a code and run, following the instructions in the tutorial.
However, the "random code" that was picked was one from the archive, associated with an unconfigured computer. So the workflow crashed.

While maybe skipping the codes that are on unconfigured computers might already help, I think the best approach is just to stop if more than one code matching is found, and prompt the user with a very clear message ("More than one code for engine quantum_espresso four: please specify which code you want to use with the -X option").

[giovannipizzi]

@sphuber @bosonie