How to get current running keyword test case name in the event (OnStopTest)

There currently, as far as I am aware, not a way to do so.

The work around is a bit labor intensive.  It's going to entail you making your keyword tests more "robust" with regards to errors where they actually log the errors directly rather than waiting for TC to do it automatically.  That way you can manually add the keyword test name to your error messages and parse it out that way.  That's the best I can do for you beyond a complete re-architecture of your automation framework.