image_qa: fix error handling

image_qa: fix error handling

Make ImageQAFailed inherit BBHandledException so exceptions raised in tests are
catched when the actual test function is executed by bb.utils.better_exec.

Change the do_image_qa tasks so errors are handled with oe.qa.handle_error. Add
some comment to explain this requires to list the test in ERROR_QA or WARN_QA.

[YOCTO #14807]
https://bugzilla.yoctoproject.org/show_bug.cgi?id=14807

Signed-off-by: Louis Rannou <louis.rannou@non.se.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>