use karma's error formatter

the error stack trace will be filtered, so that instead of timestamped
URLs it'll contain resolved source paths.
better browser names formatting

only change color of the browser name itself

the old version also colored indentation of all browser names but the
first.
fix calculating describe items' success

without this fix some items are marked as successful disregarding nested
`it` items failures.
Merge branch 'cleaner-stacktrace'