Enhance the Augeas.Error exception

Enhance the Augeas.Error exception

Augeas.Error is very limited, and only provides an error string provided
by ocaml-augeas itself: this makes it hard to actually understand what
is an Augeas.Error that was raised, and eventually perform different
actions based on that.

To overcome that, Augeas.Error is changed to be a tuple containing the
error code (as reported by Augeas), still the ocaml-augeas error string,
and error message and details of the Augeas error -- all the
informations that Augeas provides.
This breaks API compatibility with current users, but unfortunately
there is no way around that.