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
Only reports JDT aborts as ERROR if it is fatal.

Previously all JDT aborts were reported as ERROR even though it was 
almost never fatal. This made it very easy to misinterpret compile
logs. Now JDT aborts are reported as WARN when below the fatal
threshold.

Change-Id: I28ad38bac8d83105f4545d3e60d7ca8ff64c6545
Review-Link: https://gwt-review.googlesource.com/#/c/9920/