sstate: additional debugging when fetch fails occur

sstate: additional debugging when fetch fails occur

Autobuilder has been showing things like:
Checking sstate mirror object availability...ERROR: SState: cannot test file://universal/d9/bc/sstate:xz-native:x86_64-linux:5.2.5:r0:x86_64:8:d9bced04b194d5fc8d778eb8a0d674fa7375a42c8c50a9237e6d7672e9e7a00c_deploy_source_date_epoch.tar.zst: TimeoutError('timed out')
ERROR: SState: cannot test file://37/a0/sstate:libgcc-initial:core2-64-poky-linux:11.2.0:r0:core2-64:8:37a0a5aec105a0822df098f15ff2b67d0e7220204742b5d2b1f7958dda6fa5ce_deploy_source_date_epoch.tar.zst: TimeoutError('timed out')
ERROR: SState: cannot test file://universal/11/a4/sstate:libpciaccess-native:x86_64-linux:0.16:r0:x86_64:8:11a4d6c3a2e147ef7dd5f31c0ff2a91271dad49b561d8aa24849115081cf1842_deploy_source_date_epoch.tar.zst: TimeoutError('timed out')
done.

which is not helpful. To find out what really happened and where, the original
traceback is needed too.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>