StageBase::stageDirectoryExists() breaks on PreCreate

Problem/Motivation

There is a bug in StageBase::stageDirectoryExists(). The problem is that it calls getStageDirectory(), which gives you a path that may not exist if we're in the PreCreate phase of the stage life cycle.

Proposed resolution

The method should return the result of is_dir($this->getStageDirectory()). It should look, overall, like this:

    try {
      return is_dir($this->getStageDirectory());
    }
    catch (\LogicException) {
      return FALSE;
    }

A reasonable way to test this is to add a simple PreCreateEvent listener (using addTestEventListener()) which confirms that stageDirectoryExists() returns TRUE, and that the stage directory actually exists ($this->assertDirectoryExists()).