Don't catch too much in zuul_swift_upload

Avoid catching errors inside of magic libraries by not using
Except and instead checking if the 'from_file' attribute exists
to determine which library we might be using.

From feedback on
https://review.openstack.org/#/c/104411/1/modules/openstack_project/files/slave_scripts/zuul_swift_upload.py

Change-Id: If3386e5903e1fb2ff65345c4e7825ae86b7a414c