XD-1281 Create Better UI instead of Boot's default "Whitelabel Error Page"

* Add static 404 page modeled after spring.io
* Add ErrorHandler, extending `org.springframework.boot.actuate.web.BasicErrorController`
  - redirect 404 statuses to static page
Fix compiler error on using java 1.6
XD-1659 Hdfs Acceptance Tests

- hdfs Acc test will wait for file/dir to be created.

  We identified that on a fresh deployment of XD it takes longer for the hdfs to deploy.  So we give it up to 10 seconds to see if the hdfs created the directory before beginning the test.

- waitForPath now throws IllegalStateException instead of eating the Interrupted Exception.

  Also updated the test to use the results from the waitForPath in the assertTrue to determine if the directory and file exist.
XD-1618 UI: Stop JobExecution

- User can stop stoppable job execution
XD-1617 UI: StepExecution progress info

The user can view progress information about a given step

* Add Progress Bar with percentage label to the Step Execution details Screen
* Add dedicated Step Execution Progress page; show Step Execution history details (XD-1625)

Jira: https://jira.spring.io/browse/XD-1617
Remove defaults from log sink metadata description