Warn when trying to deploy over the file:/// protocol,

... instead of failing silently.

Fixes #1650.

Not necessarily the most elegant place to catch the problem ... but it works for me, and from the POV of the user, it's way better then crashing silently.
> Do you want to return here? Or just carry on running the method?

It doesn't matter. We are crashing anyway. The only thing we can do at this point is to explain the user why is this happening.