Print announce error response (#11878)

Fix HTTPFetch caller and request ID to 64 bits
Check that allocated caller ID is not DISCARD
Print body if serverlist request returns error
Don't print control characters from HTTP responses
Document special HTTPFetch caller IDs
Allow unicode to be printed

src/serverlist.cpp

@@ -97,6 +97,7 @@ void sendAnnounce(AnnounceAction action,
 	}
 
 	HTTPFetchRequest fetch_request;
+	fetch_request.caller = HTTPFETCH_PRINT_ERR;
 	fetch_request.url = g_settings->get("serverlist_url") + std::string("/announce");
 	fetch_request.method = HTTP_POST;
 	fetch_request.fields["json"] = fastWriteJson(server);