[DGD] error recovery with atomic functions

[Felix A. Croes]

I have recently committed some changes to DGD that should simplify an
old problem: how to pass back information from an atomic funtion in case
of an error.  This is of course only possible by putting a message in
the error string, but that can interfere with ordinary error interpretation
and recovery.

The error string can now be modified by the runtime_error and atomic_error
functions.  For an example of how to make use of this change, check out the
version of the kernel library integrated into

    https://github.com/dworkin/cloud-server.git

In particular, see add_atomic_message() and get_atomic_messages() in the
auto object.

Regards,
Felix Croes