enhance(error):增加两个工具异常。

exceptions/not_found.go

@@ -0,0 +1,13 @@
+package exceptions
+
+type NotFoundError struct {
+	Message string
+}
+
+func NewNotFoundError(msg string) *NotFoundError {
+	return &NotFoundError{Message: msg}
+}
+
+func (e NotFoundError) Error() string {
+	return e.Message
+}

exceptions/unsuccessful.go

@@ -0,0 +1,11 @@
+package exceptions
+
+type UnsuccessfulOperationError struct{}
+
+func NewUnsuccessfulOperationError() *UnsuccessfulOperationError {
+	return &UnsuccessfulOperationError{}
+}
+
+func (UnsuccessfulOperationError) Error() string {
+	return "Unsuccessful Operation"
+}