Add RestInterfaceSettings.errorHandler.
Add RestInterfaceSettings.errorHandler.

Enables user code to provide a custom error handling callback for writing error replies with a custom format.

Geod24/vibe.d,rejectedsoftware/vibe.d,etcimon/vibe.d,gedaiu/vibe.d,chalucha/vibe.d,etcimon/vibe.d