RFR: JShell fail over test failure not given fail over support and message confusing

[Robert Field]

All runs of JShell including all tests have fail over protection against 
launch failure -- except fail over tests, which only get one chance.

The exception thrown is only for the first failure, so for a fail over 
test with an intended first failure that is particularly confusing.

Harden the tests and provide comprehensive diagnostics.

Bugs:

   8247438: JShell: When FailOverExecutionControlProvider fails the 
proximal cause is not shown
https://bugs.openjdk.java.net/browse/JDK-8247438

   8237743: test/langtools/jdk/jshell/FailOverExecutionControlTest.java 
fails No ExecutionControlProvider with name 'nonExistent' and parameter 
keys: []
https://bugs.openjdk.java.net/browse/JDK-8237743

   8199646: JShell tests: 
jdk/jshell/FailOverDirectExecutionControlTest.java failed with 
java.lang.UnsupportedOperationException
https://bugs.openjdk.java.net/browse/JDK-8199646

Webrev:

http://cr.openjdk.java.net/~rfield/8247438v0/


Thanks,
Robert