[Develop] startup when the movies directory is gone

[Ben Dean-Kawamura]

I started refactoring our startup code today, and it all went well except for
one major snag.  How should we handle startup when the movies directory is
missing?

Our current system is to use the dialogs module, but that's frontend-specific,
so we shouldn't rely on it anymore.  I really want to keep dialogs
frontend-specific.

We can't just use a signal, because the backend code needs to know if it
should give up, or continue to start.

The way I implemented it, was to make a callback method that's called when the
backend notices the movies directory is gone.  By default, it halts the
startup process.  But frontends can install a new callback to override that
behavior.

I think that's fairly hacky, but it's the best way I could think of it.  Can
anyone think of a better way?

Ben