Make fatalError more explanatory

[rafiki270]

https://github.com/vapor/postgresql/blob/f4e7b9f70bd5383fbe1daf8f941be0a587e1290d/Sources/PostgreSQL/Message%2BParse/PostgreSQLMessageDecoder.swift#L38

Could we have something like Are you connected to PSQL and not MySQL you silly muppet? so you don't spend a day trying to fix this after a migration from MySQL :)