Be smarter about fallback projects

This is for #6505 and fixes 2 problems:
A. Try to revert to previous project instead of going straight to Getting Started folder.
B. Use Brackets source folder instead of getting caught in infinite loop when Getting Started folder has been deleted.

**Proposed Flow**:
If project fails to load, we show modal error message dialog (as we always have done), and then:

1. Get previous project path from MRU list
2.  If previous project folder does not exist, the try Getting Started folder
3. If still haven't found a project path, use Brackets source folder which is guaranteed to exist.

We do not cycle through the entire project MRU list [as proposed](https://github.com/adobe/brackets/issues/6505#issuecomment-41335593) because it's asynchronous, and reverting to previous project is probably the 80-90% case. All previous projects remain in recent project list, so user can easily select any of them.

We do not explicitly prompt -- that seems to adds more complexity to code and may confuse user even more.

After playing with code, I remembered why I did *not* add the last path (brackets source folder) to the array. It's the fail case, so we can just `resolve(FileUtils.getNativeBracketsDirectoryPath())` without verifying that path exists.

If we put it in the array, we still have to write code to handle the fail case, presumably with a comment that says "we should never get here". And path that we now exsts with get checked for existence. Ugly.