Detect channels that do not exist

If error alerts exist, they will be logged. But it is reasonable to
assume that not all errors will be fatal, so we don't necessarily quit
parsing if we find one.

This also normalises the text error of the /latest response for a
missing channel, without changing its identifier.

extractors/channel.py

@@ -22,6 +22,16 @@ def extract_channel(ucid):
 		r.raise_for_status()
 		yt_initial_data = extract_yt_initial_data(r.content.decode("utf8"))
 
+		for alert in yt_initial_data.get("alerts", []):
+			alert_text = combine_runs(alert["alertRenderer"]["text"])
+			if alert_text == "This channel does not exist.":
+				return {
+					"error": alert_text,
+					"identifier": "NOT_FOUND"
+				}
+			else:
+				print("Seen alert text '{}'".format(alert_text))
+
 		header = yt_initial_data["header"]["c4TabbedHeaderRenderer"] if "c4TabbedHeaderRenderer" in yt_initial_data["header"] else {}
 		channel_metadata = yt_initial_data["metadata"]["channelMetadataRenderer"]
 
@@ -146,7 +156,7 @@ def extract_channel_latest(ucid):
 		if r.status_code == 404:
 			cherrypy.response.status = 404
 			return {
-				"error": "Channel does not exist.",
+				"error": "This channel does not exist.",
 				"identifier": "NOT_FOUND"
 			}