oeqa/postactions: fix exception handling

oeqa/postactions: fix exception handling

The current exception handler in list_and_fetch_failed_tests_artifacts
expects a non-exisiting variable and then fail to display the original
exception message since it raises a new one. The issue has been introduced
with commit 6e80b2ab660e ("oeqa/utils/postactions: transfer whole archive
over ssh instead of doing individual copies"). Now that tests artifacts are
now handled individually, there's no point of trying to print individual
names in the exception.

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>