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.
I don't think so, because
 * The exception will only show at the console - not a real problem for the user, and there are no side-effects.
 * Even if we swallow the exception, the main problem still remains: we have half-deployed the application! The sidebar is displayed, but is not functional. That's why I am saying that we have already crashed by this point. To avoid that, we should not even try to deploy ... which would take a lot more code than this, as I described [here](https://github.com/hypothesis/h/pull/1666#issuecomment-62423059).