Make tests resilient to exceptions thrown while reporting exceptions

[nicolasstucki]

Part of #19542. We protect against the case where Exception.getMessage or Exception.getStackTrace throw an exception.

This change makes testCompilation tests/neg/i8984.scala fail as expected. tests/neg/i8984.scala is currently disabled (see #19561).

tests/neg/i8984.scala will not be fixed in this PR. Making the test framework more resilient is a priority. Fixing this is the last part of #19542.

[nicolasstucki]

This is what we get if we enable -Ydebug-cyclic to compute the stack trace of the CyclicReference

Exception thrown onComplete (probably by a reporter) in tests/neg/i8984.scala: class dotty.tools.dotc.core.CyclicReference
dotty.tools.dotc.core.CyclicReference message could not be printed due to an exception while computing the message.
dotty.tools.dotc.core.CyclicReference$.apply(TypeErrors.scala:192)              
dotty.tools.dotc.core.SymDenotations$SymDenotation.completeFrom(SymDenotations.scala:176)
dotty.tools.dotc.core.Denotations$Denotation.completeInfo$1(Denotations.scala:189)
dotty.tools.dotc.core.Denotations$Denotation.info(Denotations.scala:191)        
dotty.tools.dotc.reporting.Message$Seen.explanation(Message.scala:165)          
dotty.tools.dotc.reporting.Message$Seen.$anonfun$7(Message.scala:204)           
scala.collection.immutable.List.map(List.scala:250)                             
dotty.tools.dotc.reporting.Message$Seen.explanations(Message.scala:204)         
dotty.tools.dotc.reporting.Message.msgPostscript(Message.scala:327)             
dotty.tools.dotc.reporting.Message.message$$anonfun$1(Message.scala:372)        
dotty.tools.dotc.reporting.Message.inMessageContext(Message.scala:368)          
dotty.tools.dotc.reporting.Message.message(Message.scala:372)                   
dotty.tools.dotc.reporting.Diagnostic.message(Diagnostic.scala:102)             
dotty.tools.vulpix.ParallelTesting.showErrors$1$$anonfun$1$$anonfun$2(ParallelTesting.scala:902)
scala.collection.immutable.List.map(List.scala:250)                             
dotty.tools.vulpix.ParallelTesting.showErrors$1$$anonfun$1(ParallelTesting.scala:902)
scala.collection.immutable.List.flatMap(List.scala:293)                         
scala.collection.immutable.List.flatMap(List.scala:79)                          
dotty.tools.vulpix.ParallelTesting.dotty$tools$vulpix$ParallelTesting$NegTest$$_$showErrors$1(ParallelTesting.scala:902)
dotty.tools.vulpix.ParallelTesting$NegTest.maybeFailureMessage(ParallelTesting.scala:912)
dotty.tools.vulpix.ParallelTesting$CompilationLogic.dotty$tools$vulpix$ParallelTesting$CompilationLogic$$onComplete(ParallelTesting.scala:297)
dotty.tools.vulpix.ParallelTesting$$anon$4.checkTestSource$$anonfun$1(ParallelTesting.scala:286)
dotty.tools.vulpix.ParallelTesting$$anon$4.checkTestSource$$anonfun$adapted$1(ParallelTesting.scala:288)
scala.Function0.apply$mcV$sp(Function0.scala:42)                                
dotty.tools.vulpix.ParallelTesting$Test.tryCompile(ParallelTesting.scala:472)   
dotty.tools.vulpix.ParallelTesting$$anon$4.checkTestSource(ParallelTesting.scala:288)
dotty.tools.vulpix.ParallelTesting$Test$LoggedRunnable.run(ParallelTesting.scala:368)
dotty.tools.vulpix.ParallelTesting$Test$LoggedRunnable.run$(ParallelTesting.scala:350)
dotty.tools.vulpix.ParallelTesting$$anon$4.run(ParallelTesting.scala:283)       
java.base/java.util.concurrent.ForkJoinTask$AdaptedRunnableAction.exec(ForkJoinTask.java:1375)
java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)       
java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)        
java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)   
java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)

[odersky]

Good catch!