Fix exception handling when a concurrent backup fails

Merged Stan Hu requested to merge sh-fix-issue-241251 into master

If a concurrent backup encounters an exception, we should allow the current threads to run to completion instead of aborting the whole backup. This was causing threads to leak outside of the tests in #241251 (closed) and causing the query cache to be enabled.

This also reverts the quarantined spec in f47a2f17.

Edited by Stan Hu