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/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;
         }