question: protected CompletionStage < Result > onDevServerError ( Http . RequestHeader request , UsefulException exception ) { ObjectNode jsonError = Json . newObject ( ) ; final Throwable cause = exception . cause ; final String description = exception . description ; final String id = exception . id ; final String title = exception . title ; jsonError . put ( "description" , description ) ; jsonError . put ( "title" , title ) ; jsonError . put ( "id" , id ) ; jsonError . put ( "message" , exception . getMessage ( ) ) ; jsonError . set ( "cause" , causesToJson ( cause ) ) ; return CompletableFuture . completedFuture ( Results . internalServerError ( jsonError ) ) ; }
target:  Invoked in dev mode when a server error occurs.