This is a prototype implementation of having the ability to show an MMU error screen even for printer's errors (during an MMU operation).
Also, the retry count of unloads after failed load the extruder tube is now limited to ~3 attempts.
Technically, since this very error is not an MMU's one (MMU is just fine at this stage) but a printer's one I tried to hack the existing error-reporting infrastructure to handle such a case.
The original idea of this approach was suggested by @vintagePC
D.R.racer
3d-gussner