Check for FSensor OFF after unloaded to FINDA

we may discuss when to check for the FSensor state and why,
but this approach seemed to me like the least invasive method
of just checking the fsensor state and reporting an error in case
it didn't switch off.

src/logic/unload_filament.cpp

@@ -43,6 +43,11 @@ bool UnloadFilament::StepInner() {
                 ml::leds.SetMode(mg::globals.ActiveSlot(), ml::green, ml::off);
                 ml::leds.SetMode(mg::globals.ActiveSlot(), ml::red, ml::blink0);
                 mi::idler.Disengage();
+            } else if (mfs::fsensor.Pressed()) {
+                // fsensor still pressed - that smells bad - a piece of filament may still be present in the heatsink
+                // and that would cause serious problems while loading another filament
+                state = ProgressCode::ERRDisengagingIdler;
+                error = ErrorCode::FSENSOR_DIDNT_SWITCH_OFF;
             } else {
                 state = ProgressCode::RetractingFromFinda;
                 retract.Reset();