Summary:
Cleans up the `ExtendedError` internal type and moves it into a separate module instead of burying it in `parseErrorStack.js`.
Also, this resolves some unnecessary Flow type suppressions.
Changelog:
[Internal]
Reviewed By: GijsWeterings
Differential Revision: D28470299
fbshipit-source-id: 04093243f06f67f41567270ef9778f01c7549b05