Improve deadlock error handling

[eileenmcnaughton]

This simply captures the error when the deadlock is first hit & then if the deadlock threshold it hit, causing an abort, it re-throws the original error - so the backtrace is retained.

We have been running this in production for about 6 months and the errors have been more meaningful

(@seamuslee001 you might have an interest)

[civibot]

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[seamuslee001]

Change looks fine to me and unit test have passed so merging