Default exception handler should exit on exception if exitCode was set

[remkop]

When a runtime exception occurs while executing one of the convenience methods (CommandLine.run, CommandLine.call, CommandLine.parseWithHandlers, ...) the exception is wrapped in a ExecutionException and passed to the exception handler.

Currently, the default exception handler will rethrow this ExecutionException unconditionally.

However, when an exit code is set on the exception handler, the expectation is that picocli calls System.exit with that error code when an exception is encountered.