Add extra failure error messages

src/Backends/GLFW3/Misc.cpp

@@ -154,8 +154,10 @@ BOOL Backend_Init(void)
 {
 	if (glfwInit() == GL_TRUE)
 		return TRUE;
-	else
-		return FALSE;
+
+	Backend_ShowMessageBox("Fatal error", "Could not initialise GLFW3");
+
+	return FALSE;
 }
 
 void Backend_Deinit(void)

src/Backends/SDL2/Misc.cpp

@@ -49,9 +49,13 @@ BOOL Backend_Init(void)
 			return TRUE;
 		}
 
+		Backend_ShowMessageBox("Fatal error", "Could not initialise SDL2 video subsystem");
+
 		SDL_Quit();
 	}
 
+	Backend_ShowMessageBox("Fatal error", "Could not initialise SDL2");
+
 	return FALSE;
 }