Watch files which fail before emitting a resource

Regression introduced by the CSS watching code (specifically, f230eba62)
by the sourceIsWatched check. We need to be able to tell the difference
between "source handler didn't do anything because there was an error"
and "source handler didn't do anything because it's web-specific and
this is an os arch".

A simple fix would have been to interpret compileStep.error as
"sourceIsWatched = true", but I didn't think of that until after doing
it the slightly more complicated but more precise way :)

Also, ensure that if the runner rebuilds the client and there's an
error, it properly kills the app process (and the watchers and the
keepalive interval, etc).