Refactors c.g.gwt.junit to use common SerializableThrowable & StacktraceDeobfuscator.

- Gets rid of duplicate code in junit for deobfuscation of stack traces
- Improves the messaging of unserializable exceptions
- Minor improvements to the reporting of test infra failures

Change-Id: I1e1021bc99ac88ea6d9d47c3d23c83e79a896213
Review-Link: https://gwt-review.googlesource.com/#/c/2290/

Review by: skybrian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11581 8db76d5a-ed1c-0410-87a9-c151d255dfc7