add simple test as example
report silent HTML parsing errors

Otherwise, silently modified HTML causes mysterious exceptions.
cleanup, move stuff from cli to SiteDiff.initialize
simplify queueing logic

The flow of control is now:
  SiteDiff.run() -> SiteDiff.queue_read() -> UriWrapper.queue()
an empty HTTP error message should not crash sitediff

+ more cleanup
default to not showing tons of HTML parsing "errors"