When a connection is in asynchronous mode, failing operations (search,

When a connection is in asynchronous mode, failing operations (search,
	scan, etc.) no longer throw exceptions.  This is because the
	event-loop in asychronous applications needs to check for
	errors anyway, in case the server has returned high-level
	diagnostics such as "unsupported use attribute".  So this
	check within the event-loop is now the only place where
	error-checking need be done.
Add new public ZOOM::Connection() function, check(), which throws an
	exception if an error is pending (even if the connection is
	asynchronous).
New YAZ logging level, "zoom_check", notes when errors are detected
	and either left unreported (asynchronous connections) or
	thrown as exception (synchronous).