When CPU core is invalid, fall back to JIT instead of interpreter

This might happen if someone moves settings between e.g. a PC and
an Android device, or if someone was using JITIL and updates Dolphin.

I also made the panic alert a bit more explanatory.

Source/Core/Core/PowerPC/JitInterface.cpp

@@ -64,7 +64,9 @@ CPUCoreBase* InitJitCore(int core)
     break;
 
   default:
-    PanicAlert("Unrecognizable cpu_core: %d", core);
+    PanicAlertT("The selected CPU emulation core (%d) is not available. "
+                "Please select a different CPU emulation core in the settings.",
+                core);
     g_jit = nullptr;
     return nullptr;
   }