Add new API function ZOOM_diag_str(), which maps error code to

Add new API function ZOOM_diag_str(), which maps error code to
human-readable message.  It uses code lifted from ZOOM_connection_error()
(which now calls it) to map the ZOOM-specific errors by hand, and fall
back to diagbib1_str() for others.  I did this because I need the new
function in ZOOM-C++, for reasons which would be otiose, for the moment,
to rehearse.