After commit 905e224849fbbed1719e0add231b00e2d570b3b4 (image_qa: fix
error handling), any unexpected exceptions in do_image_qa() would result
in a variable being set, but never used, effectively hiding the error.

Since image_qa now calls oe.qa.exit_if_errors(), remove the support for
oe.utils.ImageQAFailed and instead rely on the called functions to call
oe.qa.handle_error() themselves. This matches what do_package_qa() does.

Also update the description of do_image_qa() to explain that the called
functions are expected to call oe.qa.handle_error() themselves.

[ YOCTO #15601 ]

(From OE-Core rev: a1b28a88bc7697371ab166b18587b615d6d39c8e)

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 0c3e111c965af2bc56533633c376b70b7fa5e1de)
Signed-off-by: Steve Sakoman <steve@sakoman.com>