bitbake: lib/bb/event: improve handling of event queue on exit

bitbake: lib/bb/event: improve handling of event queue on exit

If BitBake exits before a UI handler (server) has been registered, we
print the event queue; if there are any errors or other non-debug
messages just print these and suppress the rest of the message queue.

This improves the output when sanity check failures occur with OE-Core
by avoiding printing a long stream of uninformative debug messages.

(Bitbake rev: 8668a94cb1841798636b68fe123400d6b81f6574)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>