Crawl https URLs, too

Seems I don't need to do anything for them - they just work?
Added modifyMV helper function

...makes it easier to update values stored in MVars
Guard most of workerThread against exceptions

Not sure it can happen, but why take a risk.
Removed debug output & some uneeded comments

Maybe I'm getting too bold, but I think some of these comments are
either unneeded, or they can be replaced by calling a function with
a descriptive name.