RFR JDK-8131915: CompletionFailure during import listing crashes javac

[Jan Lahoda]

Hello,

This is a fix for:
https://bugs.openjdk.java.net/browse/JDK-8131915

which Liam reported recently. The problem is that if a CompletionFailure 
happens while listing import content (e.g. due to a missing supertype), 
it is not handled properly and causes a javac crash.

The proposed fix is to catch the CompletionFailure and handle it 
properly (log an error).

A webrev with the fix is here:
http://cr.openjdk.java.net/~jlahoda/8131915/webrev.00/

Any feedback is welcome,

Thanks!
     Jan