Improve install instructions for better problem avoidance
Things added/changed:
- describe how to find the UnitTesting quark and manually delete it if need be.
- describe steps in such a way that users might avoid the UnitTesting issue altogether.
- don't suggest installing quarks via GUI (will likely end up with a broken class library if installing via GUI).
A minor typo fix got included in this MR for some reason. Oops!