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/d3d12/SDL_gpu_d3d12.c

@@ -8686,7 +8686,7 @@ static bool D3D12_PrepareDriver(SDL_VideoDevice *_this, SDL_PropertiesID props)
     bool xr = SDL_GetBooleanProperty(props, SDL_PROP_GPU_DEVICE_CREATE_XR_ENABLE_BOOLEAN, false);
     if (xr) {
         if (!SDL_OpenXR_LoadLibrary()) {
-            SDL_LogWarn(SDL_LOG_CATEGORY_GPU, "D3D12: Failed to load OpenXR");
+            SDL_LogWarn(SDL_LOG_CATEGORY_GPU, "D3D12: Failed to load OpenXR: %s", SDL_GetError());
             return false;
         }
 

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;
         }

src/gpu/xr/SDL_openxrdyn.c

@@ -259,7 +259,10 @@ SDL_DECLSPEC bool SDLCALL SDL_OpenXR_LoadLibrary(void)
         }
 
         if (!openxr_loader.lib) {
-            SDL_SetError("Failed loading OpenXR library");
+            SDL_SetError("Failed to load OpenXR loader library. "
+                         "On Windows, ensure openxr_loader.dll is in your application directory or PATH. "
+                         "On Linux, install the OpenXR loader package (libopenxr-loader) or set LD_LIBRARY_PATH. "
+                         "You can also use the SDL_HINT_OPENXR_LIBRARY hint to specify the loader path.");
             openxr_load_refcount--;
             return false;
         }