Add explanation in assertion

read_mattermost.py

@@ -72,8 +72,8 @@ mm = mattermost.MMApi(f"https://{SERVER}/api")
 if TOKEN:
 	mm.login(bearer=TOKEN)
 else:
-	assert USER
-	assert PASSWORD
+	assert USER, "USER and PASSWORD or TOKEN envvars must be set"
+	assert PASSWORD, "USER and PASSWORD or TOKEN envvars must be set"
 	mm.login(USER, PASSWORD)
 
 # pylint: disable=protected-access # bad library design: can't fix this without extra API request