Report a proper error when the JDT compiler aborts.

That is, print the file and line where the internal error happened, instead of
spurious, nondeterministic errors about being unable to find java/lang/Object
and so on. Once the file is identified it's possible to work around it;
sometimes GWT doesn't need to compile the file at all.

Also added JavaDoc on some methods.

Change-Id: I74397bfa2be4e75ec116f42611197a7fa3953701
Review-Link: https://gwt-review.googlesource.com/#/c/2090/

Review by: mdempsky@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11542 8db76d5a-ed1c-0410-87a9-c151d255dfc7