users guide: Add documentation for custom compile-time errors
users guide: Add documentation for custom compile-time errors

shlevy/ghc,nushio3/ghc,snoyberg/ghc,shlevy/ghc,gridaphobe/ghc,oldmanmike/ghc,vikraman/ghc,sgillespie/ghc,vTurbine/ghc,sgillespie/ghc,gridaphobe/ghc,shlevy/ghc,vTurbine/ghc,tjakway/ghcjvm,oldmanmike/ghc,sgillespie/ghc,shlevy/ghc,sgillespie/ghc,vTurbine/ghc,mettekou/ghc,sdiehl/ghc,vikraman/ghc,snoyberg/ghc,tjakway/ghcjvm,gridaphobe/ghc,olsner/ghc,sgillespie/ghc,sgillespie/ghc,oldmanmike/ghc,vikraman/ghc,nushio3/ghc,vTurbine/ghc,mcschroeder/ghc,snoyberg/ghc,ezyang/ghc,nushio3/ghc,oldmanmike/ghc,olsner/ghc,mcschroeder/ghc,sdiehl/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,vTurbine/ghc,mettekou/ghc,shlevy/ghc,nushio3/ghc,shlevy/ghc,sdiehl/ghc,vikraman/ghc,gridaphobe/ghc,oldmanmike/ghc,tjakway/ghcjvm,mettekou/ghc,olsner/ghc,snoyberg/ghc,tjakway/ghcjvm,nushio3/ghc,olsner/ghc,shlevy/ghc,tjakway/ghcjvm,ezyang/ghc,vikraman/ghc,nushio3/ghc,mettekou/ghc,mettekou/ghc,mcschroeder/ghc,mcschroeder/ghc,oldmanmike/ghc,tjakway/ghcjvm,gridaphobe/ghc,sgillespie/ghc,vTurbine/ghc,vTurbine/ghc,sdiehl/ghc,mcschroeder/ghc,snoyberg/ghc,ezyang/ghc,snoyberg/ghc,olsner/ghc,gridaphobe/ghc,vikraman/ghc,ezyang/ghc,vikraman/ghc,nushio3/ghc,ezyang/ghc,sdiehl/ghc,mettekou/ghc,mcschroeder/ghc,olsner/ghc,mcschroeder/ghc,gridaphobe/ghc,snoyberg/ghc,oldmanmike/ghc,sdiehl/ghc,mettekou/ghc,sdiehl/ghc,tjakway/ghcjvm