Clean up GHC's error reporting

  - the GhcException type has some more constructors: CmdLineError,
    ProgramError, and InstallationError.  OtherError has gone.

  - most error messages should begin with "<location>:".  When the
    error is on the command-line or in GHC itself, <location> is
    "ghc", for consistency with std Unix semantics.