sstate: Improve failure to obtain archive message/handling
The bb.fatal() case where sstate failed to find/use an archive in setcene tasks
is suboptimal. Bitbakes handling of setscene tasks will be to warn but the fatal
will turn this into an error, despite the real task being rerun.
In these failure cases other messages would usually have been printed so turn
this into a warning and raise a handled exception status so that bitbake knows
to fail the task but not print more messages.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
