MMU: Improve unload handling when print is aborted

- Don't allow re-heating the nozzle or the MMU to unload the filament if the print was aborted by a temperature error
- Allow MMU to unload filament if there is no temperature error, and there is no fan error when Fan check is enabled
- When Fan check is disabled, the MMU may unload if there is no temperature error

Change in memory:
Flash: +14 bytes
SRAM: 0 bytes

PFW-1557

Firmware/ultralcd.cpp

@@ -5716,10 +5716,12 @@ void lcd_print_stop_finish()
 
         // restore the auto hotend state
         hotendDefaultAutoFanState();
-    }
 
-    if (MMU2::mmu2.Enabled() && MMU2::mmu2.FindaDetectsFilament())
-    {
+        if (MMU2::mmu2.Enabled() && MMU2::mmu2.FindaDetectsFilament()
+#ifdef FANCHECK
+            && fan_check_error != EFCE_REPORTED
+#endif //FANCHECK
+        ) {
             // The print was aborted while when the nozzle was cold:
             //     1. in a paused state                      => a partial backup in RAM is always available
             //     2. after a recoverable thermal/fan error had paused the print => only extruder temperature is saved to RAM
@@ -5735,6 +5737,7 @@ void lcd_print_stop_finish()
                 MMU2::mmu2.unload(); // M702
             }
         }
+    }
 
     lcd_cooldown(); //turns off heaters and fan; goes to status screen.