Various small cleanups - largely docs

- Fix & update documentation in a bunch of places
- Add a fail-fast warning when missing DOM id will cause bottom panels to not get created correctly (per #6252)
- Add extra FileSystem unit tests around the fs root
- Fix bits of the ProjectManager exclusion regex that were missing trailing "$" (I verified these are all full names, not just prefixes) -- and add WebStorm project state as an excluded item
- Slightly better messages when unit tests fail due to a Promise resolution going the wrong way
- Fix some minor JSLint errors