enhancesGrails.groovy

import grails.artefact.Enhances
import org.grails.core.artefact.ControllerArtefactHandler

@Enhances(ControllerArtefactHandler.TYPE) //you can do the same with ServiceArtefactHandler.TYPE
trait ExceptionHandler {

    def handleControllerError(Exception e) {
        response.status = 400
        render ApiError.unknownError as JSON
    }
}