Reorganize errors

common/errors/errors.go

@@ -0,0 +1,41 @@
+package errors
+
+import (
+	"fmt"
+
+	"gemini-grc/errors"
+)
+
+// HostError is an error encountered while
+// visiting a host, and should be recorded
+// to the snapshot.
+type HostError struct {
+	Err error
+}
+
+func (e *HostError) Error() string {
+	return e.Err.Error()
+}
+
+func (e *HostError) Unwrap() error {
+	return e.Err
+}
+
+func NewHostError(err error) error {
+	return &HostError{Err: err}
+}
+
+func IsHostError(err error) bool {
+	if err == nil {
+		return false
+	}
+	var asError *HostError
+	return errors.As(err, &asError)
+}
+
+// Sentinel errors used for their string message primarily.
+// Do not use them by themselves, to be embedded to HostError.
+var (
+	ErrBlacklistMatch = fmt.Errorf("black list match")
+	ErrRobotsMatch    = fmt.Errorf("robots match")
+)