Make ExceptionCause public

[pgmillon]

Useful to cleanly break from a retry wrapper, keeping the root exception:

retry(50) {
  try {
    if(!ec2GetTags(instanceId).containsKey('slave_configuration')) {
      sleep 30
      throw new Exception('Missing tag slave_configuration on instance')
    }
  } catch (AmazonServiceException e) {
    throw new FlowInterruptedException(Result.FAILURE, new ExceptionCause(e) {})
  }
}

[jglick]

Would rather move this to Jenkins core, as noted in Javadoc.

Anyway the script mentioned in the PR description is not sandbox-safe, so we do not want to encourage such idioms.

[jglick]

(also see #124)