assert failures are not properly logged

[FeepingCreature]

When a vibe.d request handler is terminated with an AssertError, the backtrace is logged but not the message, and it is logged later than the actual problem occurred.

[Geod24]

Vibe used to catch assert, but that was deprecated a while ago. In general, after an assert is triggered, the program is in an undefined state, so I don't think Vibe should take any action to handle those. See #666 for the discussion.

Feel free to comment / re-open if I missed anything.