debug: Send flow tree dump to stderr for consistency
Fail the script task when page URL load fails

It's confusing that specifying a nonexistent file on the command line just
gives me a blank page.  Until we have a real 404 page let's just crash.
Reflow and redisplay after script timers fire

We don't know what the script changed.  This will be less painful with
incremental layout.

Fixes a crash from calling reflow() with self.damage = None.
auto merge of #619 : kmcallister/servo/script-redisplay, r=jdm

Stuff I found in the process of working on incremental layout.