Clean up GHC's error reporting.

  - the GhcException type has some more constructors: CmdLineError,
    UserError, 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.

  - GHCi no longer prints a superfluous "ghc: " before certain error
    messages.