stagefailed is more useful

- Can now apply to normal stages, simply defaults to "false" in normal stages.
- Post-level cutscenes are now always skipped when the stage was failed.
- Exposed the boolean as a Lua read+write global.

Desired for SUGOI, as it allows for visited flags not be updated, and level completion emblems to not be awarded. Which means a lot less crappy non-ideal workarounds.

Normal stage intermission currently does not reflect failure state at all. Maybe it could always skip, never award score bonuses, have different text... etc. Probably would leave that up to vanilla dev opinion.