RCG: Need good error message when FIR filter used in fast model

FIR filters are allowed only in 2k and 4k models, but if they are added to a faster model, the result is a cryptic "firNum" undefined message.

A nice error message would be better.

added RCG label

@dripta.bhattacharjee @madeline.wade @joseph.betzwieser

changed milestone to %advligorts 5.0.0

changed milestone to %advligorts 5.1.0

mentioned in merge request !385 (merged)

Fixed in !385 (merged).

closed