Event: Implement an error queue to handle system errors.

The error events have higher priority than the normal events as
they will be returned to userspace first. It is then up to userspace on
how to handle those system errors.

Often times, these system errors would otherwise cause kernel panics.

Originally pa/1741497.

Bug: 162036180
Change-Id: If9a0310a15c55e7e1e299bdcda7790d9300801db
Signed-off-by: Ge Bian <bian@google.com>