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.
> [...]  if you deal with a potential exception, generally you don't want to leave the program running just for fun. Also It'll cause false negatives should we ever deploy any kind of top level error reporting.

OK, that makes sense; we are now avoiding the exception in https://github.com/hypothesis/h/commit/29599c71bfa20fdb7be3898b6c2fb6111c3b6942.