obs-x264: Convey lack of Rec. 2100 support

plugins/obs-x264/data/locale/en-US.ini

@@ -10,3 +10,4 @@ Tune="Tune"
 None="(None)"
 EncoderOptions="x264 Options (separated by space)"
 VFR="Variable Framerate (VFR)"
+HdrUnsupported="OBS does not support using x264 with Rec. 2100"

plugins/obs-x264/obs-x264.c

@@ -701,6 +701,8 @@ static void *obs_x264_create(obs_data_t *settings, obs_encoder_t *encoder)
 	switch (voi->colorspace) {
 	case VIDEO_CS_2100_PQ:
 	case VIDEO_CS_2100_HLG:
+		obs_encoder_set_last_error(encoder,
+					   obs_module_text("HdrUnsupported"));
 		warn_enc(encoder,
 			 "OBS does not support using x264 with Rec. 2100");
 		return NULL;