[Libguestfs] [PATCH nbdkit 0/3] Send informational error messages to NBD client

Currently nbdkit only sends an error code (like NBD_REP_ERR_*) and possibly an errno (NBD_E*) back to the NBD client. This loses a lot of useful information, meaning callers can only find out what really went wrong by looking at server-side logs, which of course might not be available or easy to access. This change sends informational error messages back to the NBD client ... in most cases. There are some caveats, see patch 3/3. Rich.