Fix how the promises are created and handle them properly in the tests

[Elgolfin]

• Catch the possible errors from the find library (find.file method)
  • With the built-in error handler for the async method
  • With a try/catch for the sync method
  • Properly reject the generated promise with the given error (if any)
• Fix the unit tests to properly handle the promises following the below guidelines
  https://www.sitepoint.com/promises-in-javascript-unit-tests-the-definitive-guide/
  • Add Chai As Promised to do so
  • Remove all done() callbacks (useless)
  • Add a unit test to handle an error when the root directory does not exist

[coveralls]

Coverage remained the same at 100.0% when pulling 460a25e on Elgolfin:pull-request-promises-management into 49efda7 on kaesetoast:master.

[kaesetoast]

@Elgolfin thanks a lot! :-)

[Elgolfin]

You are very welcome.