Improve OpenXR loader error messages

When the OpenXR loader library fails to load, provide a detailed error
message explaining:
- On Windows: need openxr_loader.dll in app directory or PATH
- On Linux: install libopenxr-loader package or set LD_LIBRARY_PATH
- Can use SDL_HINT_OPENXR_LIBRARY hint to specify path

Also update D3D12 backend to include the error in its warning log,
and remove redundant SDL_SetError in Vulkan backend that was
overwriting the detailed message.

src/gpu/vulkan/SDL_gpu_vulkan.c

@@ -12738,7 +12738,7 @@ static bool VULKAN_PrepareDriver(SDL_VideoDevice *_this, SDL_PropertiesID props)
 
     if (xr) {
         if (!SDL_OpenXR_LoadLibrary()) {
-            SDL_SetError("Failed to load OpenXR loader or a required symbol");
+            /* SDL_GetError() already has the detailed message from SDL_OpenXR_LoadLibrary() */
             SDL_Vulkan_UnloadLibrary();
             return false;
         }