qemu: monitor: Extract handling of JSON block job error codes

qemu: monitor: Extract handling of JSON block job error codes

My intention is to split qemuMonitorJSONBlockJob() into simpler separate
functions for every block job type. Since the error handling code is the
same for all block jobs, this patch extracts the code into a separate
function that will later be reused in more places.

With the new helper qemuMonitorJSONErrorIsClass we can save a few
function calls as we can extract the error object once.