@Override
    protected String createErrorMessage(ApiRequest request, Throwable error) {
        if (error.getMessage() == null) {
            return error.getClass().getCanonicalName();
        }
        return error.getMessage();
    }
@see io.apiman.gateway.engine.impl.DefaultPolicyErrorWriter#createErrorResponse(java.lang.Throwable)
gateway/engine/core/src/main/java/io/apiman/gateway/engine/impl/TracePolicyErrorWriter.java