This worked for me. For future reference in case anyone else is using this with Repetier-Server, the pause.g action did not execute even after the change to my M591 command, but it did send a different notification that I was able to catch via Repetier's G-codes response to event and send the pause command from there. I used this regex in Repetier.
^Filament\serror\son\sextruder\s\d:.*