cubicweb #4491179 do not swallow postcommit errors [deprecated]

As of today, exceptions signaled from a postcommit_event are swallowed (though a "critical" warning is issued).

I contend that it is harmful to do this, since the postcommit_event is not just a place to perform unimportant work (such as email notifications or janitorial file system cleanup) but the only way to e.g. chain transactions. I can't see a deep reason why crashes (read: programming errors) in follow-up transactions should be swallowed. Contrarily, the current behaviour makes it painful to debug such situations.

With respect to the transaction status (committed ? uncommitted ?) we can provide the information in a wrapper exception or add to the current exception, even though it would be redundant since crashing in postcommit_event currently means, by design, that the database transaction is committed.

done in3.21.0
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