Super Dev Mode: fix error log for invalid .gwt.xml files

Makes sure that lastBuild is always set regardless of the
exception, so the error log will show up.

Bug: issue 8966
Change-Id: Ibf767a09be919759a9fbaebe71d5d0a663e9cf89
(cherry picked from commit 56dd298fa5d815bbb77e3327de0f80d7cce979b8)