RFR 8188072: JShell: NPE in SourceCodeAnalysis.completionSuggestions()

[Robert Field]

JDK-8190510 has been filed to diagnose and address the underlying cause 
of this error.

The more general and important fix is to assure that failures of this 
class do not crash the jshell tool -- so this addresses that.

Catching this in SourceCodeAnalysisImpl.completionSuggestions() causes 
it move on to the same failure in SourceCodeAnalysisImpl.documentation().

Both now catch all Throwables, reporting the exception when debugging is 
on ("/debug", see JDK-8190510) and returning the empty list (the no 
result value).

Bug:

     https://bugs.openjdk.java.net/browse/JDK-8188072

Webrev:

     http://cr.openjdk.java.net/~rfield/8188072v0.webrev/

-Robert