Maybe someone else has a solution but I cannot find one at the moment with DQL. Metrics on Grail is not quite GA so there are a lot of missing metrics. For a full list of available ones, you can find that here: Built-in metrics on Grail | Dynatrace Docs
Using the classic metric selector, to do what you're looking for I'd make the request associated with checking out a key request which would allow you to graph the errors using the following metric keys.
Also regarding SRG, it supports SLOs which support the classic metric selector.
Hope this helps!