The ->send method is expected to set errno.  We didn't set it
explicitly before, although it was usually set implicitly.  Let's
improve this.

Firstly never lose the real error message (from gnutls) so always call
nbdkit_error (... gnutls_strerror ...)  along error paths, even though
that sometimes means we'll get double errors because the caller will
also call nbdkit_error (... "%m" ...).

Secondly make sure errno is set to something on error.  Surround each
gnutls call with errno = 0; ... if (errno == 0) errno = EIO.  This
means we'll capture the underlying syscall error if there was one, but
if there wasn't one then we'll always return with errno set on error
paths out of the function.