Currently, when browsertime fails, we fail with very generic messages like "no results found". Instead we should be pulling them from the browsertime.json file.

This patch adds a line handler to the mozprocess object responsible for running browsertime.

The line handler strips away some unnecessary information the log, and then outputs it using Raptor's log handler (level determined by browsertime log message). If a line that is prefixed with error we immediately raise an exception to kill the process handler, and then raise the failure in Raptor to prevent results processing.

https://hg.mozilla.org/mozilla-central/rev/ace557885b11