Add MSG_THERMAL_RUNAWAY_BED (#9691)

Marlin/src/lcd/language/language_en.h

 #ifndef MSG_THERMAL_RUNAWAY
   #define MSG_THERMAL_RUNAWAY                 _UxGT("THERMAL RUNAWAY")
 #endif
+#ifndef MSG_THERMAL_RUNAWAY_BED
+  #define MSG_THERMAL_RUNAWAY_BED             _UxGT("BED THERMAL RUNAWAY")
+#endif
 #ifndef MSG_ERR_MAXTEMP
   #define MSG_ERR_MAXTEMP                     _UxGT("Err: MAXTEMP")
 #endif

Marlin/src/module/temperature.cpp

               _temp_error(hotend, PSTR(MSG_T_HEATING_FAILED), PSTR(MSG_HEATING_FAILED_LCD));
           }
           else if (current < target - (MAX_OVERSHOOT_PID_AUTOTUNE)) // Heated, then temperature fell too far?
-            _temp_error(hotend, PSTR(MSG_T_THERMAL_RUNAWAY), PSTR(MSG_THERMAL_RUNAWAY));
+            _temp_error(hotend, PSTR(MSG_T_THERMAL_RUNAWAY),
+              hotend >= 0 ? PSTR(MSG_THERMAL_RUNAWAY) : PSTR(MSG_THERMAL_RUNAWAY_BED)
+            );
         #endif
       } // every 2 seconds
 
         else if (PENDING(millis(), *timer)) break;
         *state = TRRunaway;
       case TRRunaway:
-        _temp_error(heater_id, PSTR(MSG_T_THERMAL_RUNAWAY), PSTR(MSG_THERMAL_RUNAWAY));
+        _temp_error(heater_id, PSTR(MSG_T_THERMAL_RUNAWAY),
+          heater_id >= 0 ? PSTR(MSG_THERMAL_RUNAWAY) : PSTR(MSG_THERMAL_RUNAWAY_BED)
+        );
     }
   }