Better error message when FFmpeg doesn't support CUDA (#499)

Author: NicolasHug

src/torchcodec/decoders/_core/CudaDevice.cpp

@@ -98,10 +98,14 @@ AVBufferRef* getFFMPEGContextFromExistingCudaContext(
       nullptr,
       AV_CUDA_USE_CURRENT_CONTEXT);
   if (err < 0) {
+    /* clang-format off */
     TORCH_CHECK(
         false,
-        "Failed to create specified HW device",
-        getFFMPEGErrorStringFromErrorCode(err));
+        "Failed to create specified HW device. This typically happens when ",
+        "your installed FFmpeg doesn't support CUDA (see ",
+        "https://github.com/pytorch/torchcodec#installing-cuda-enabled-torchcodec",
+        "). FFmpeg error: ", getFFMPEGErrorStringFromErrorCode(err));
+    /* clang-format on */
   }
   return hw_device_ctx;
 }