Prim.hs: add some ACKs.
Prim.hs: add some ACKs.

Added FileNotFound and FileExists, but I haven't added support for
parsing them as they need to know about the files in question, just
as UnknownCommand does.
I think perhaps we should just parse the prefix to determine the
type of error and then return this type along with the raw ack until
we can reliably discern those extra bits of information that make the
errors more descriptive while still retaining the usefulness of
being able to identify different errors via proper types.

darcs-hash:20070825111639-ea16c-ecd1b83603e68cbcb721f6e89005f669aad828a0.gz

beni55/libmpd-haskell,sol/libmpd-haskell