Minor additions to the spec

[Tom Gundersen]

Hi guys,

David and I have recently been working on a dbus project (more on that
in a separate email), and in that context have been spending quite some
time figuring out things that the reference implementation does that are
not documented in the spec.

I'd like to work a bit on fleshing out the spec with the missing bits, so
these are the first few changes to get the ball rolling.

The main purpose of these patches is to document the error codes a compliant
implementation must return in various expected error cases (limits exceeded,
permission denied, invalid request, etc). I still left any internal errors
(out of memory etc) or configuration errors undocumented as that does not
seem like something for clients to rely upon or handle.

Cheers,

Tom