[errors] remove "is_handled" logic, turn unhandled into anomalies

We place the check for unhandled exceptions in the `is_anomaly`
function, and consider all the exceptions non-handled by the printers
always anomalies.

This reworks the solution implemented in
ea3909466eaaf86ff212c0a002e5df11e4a979f5 , in particular
`allow_uncaught` cannot be used anymore, all exceptions must install a
printer.

In order to pass the test-suite CI we also had to register some
printers, that were not registered for no reason, forcing clients to
call a post-processing step on errors.

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