obs-ffmpeg: Show error if trying to use AV1 fallback

plugins/obs-ffmpeg/data/locale/en-US.ini

@@ -36,6 +36,7 @@ NVENC.I010Unsupported="NVENC does not support I010. Use P010 instead."
 NVENC.10bitUnsupported="Cannot perform 10-bit encode on this encoder."
 NVENC.16bitUnsupported="Cannot perform 16-bit encode on this encoder."
 NVENC.TooManyBFrames="Max B-frames setting (%d) is more than encoder supports (%d)."
+NVENC.NoAV1FallbackPossible="AV1 encoding is not available with the current settings. Try disabling any re-scaling or GPU options that may be set. Check the log for more details."
 NVENC.Preset2.p1="P1: Fastest (Lowest Quality)"
 NVENC.Preset2.p2="P2: Faster (Lower Quality)"
 NVENC.Preset2.p3="P3: Fast (Low Quality)"

plugins/obs-ffmpeg/jim-nvenc.c

@@ -1165,6 +1165,11 @@ reroute:
 	case CODEC_HEVC:
 		return obs_encoder_create_rerouted(encoder,
 						   "ffmpeg_hevc_nvenc");
+	case CODEC_AV1:
+		obs_encoder_set_last_error(
+			encoder,
+			obs_module_text("NVENC.NoAV1FallbackPossible"));
+		break;
 	}
 
 	return NULL;