Display UI build failures in browser

Instead of crashing the Jetty daemon when there is a UI build failure,
keep the daemon running and display the output from Buck in the web
browser.  This allows for a slightly quicker fix and retry cycle,
especially for minor typos in ui.xml files.  Simply edit the offending
source and reload, without needing to relaunch the daemon.

Change-Id: Iac31374d4efd44c4c6a58ce08497fbf24dd6dfa9