See #19905 for context.
This WIP MR does another pass over DriverUnknownMessages left in GHC and tries to port them. In particular, I have spotted a DriverUnknownMessage in parser' code which I think was an oversight, and I have now fixed it.
There are a bunch of DriverUnknownMessages which uses the cannotFindModule function from GHC.Iface.Errors, but I don't want to touch it because there are other tickets being worked on by other people which will change this, and I don't want to step on their toes.