Fix null response for instance stoppable check when connection refused.

[huizhilu]

Issues

• My PR addresses the following Helix issues and references them in the PR title:
  Null returned for instance stoppable check when connection refused #503 Null returned for instance stoppable check when connection refused

Description

• Here are some details about my PR, including screenshots of any UI changes:
  (Write a concise description including what, why, how)
  instance stoppable check endpoint /clusters/<cluster>/instances/<instanceId>/stoppable return null when connection between helix rest server and storage node.
  We return a StoppableCheck object when connection refused.

Tests

• The following tests are written for this issue:
  (List the names of added unit/integration tests)
  testGetInstanceStoppableCheckConnectionRefused

• The following is the result of the "mvn test" command on the appropriate module:
  (Copy & paste the result of "mvn test")

[INFO] Tests run: 86, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 23.438 s - in TestSuite
[INFO]
[INFO] Results:
[INFO]
[INFO] Tests run: 86, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 30.896 s
[INFO] Finished at: 2019-10-03T19:13:15-07:00
[INFO] ------------------------------------------------------------------------

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  Subject is separated from body by a blank line
  Subject is limited to 50 characters (not including Jira issue reference)
  Subject does not end with a period
  Subject uses the imperative mood ("add", not "adding")
  Body wraps at 72 characters
  Body explains "what" and "why", not "how"
  Documentation

  In case of new functionality, my PR adds documentation in the following wiki page:
  (Link the GitHub wiki you added)

Code Quality

• My diff has been formatted using helix-style-intellij.xml

[huizhilu]

This PR is ready to be merged, approved by @dasahcc

Final commit message:
Issue: Instance stoppable check endpoint /clusters//instances//stoppable returns null when connection between helix rest server and storage node.
This diff fixes this by: return a StoppableCheck object when connection refused.