refactor and organize code

internal/gperr/error.go

@@ -0,0 +1,33 @@
+package gperr
+
+type Error interface {
+	error
+
+	// Is is a wrapper for errors.Is when there is no sub-error.
+	//
+	// When there are sub-errors, they will also be checked.
+	Is(other error) bool
+	// With appends a sub-error to the error.
+	With(extra error) Error
+	// Withf is a wrapper for With(fmt.Errorf(format, args...)).
+	Withf(format string, args ...any) Error
+	// Subject prepends the given subject with a colon and space to the error message.
+	//
+	// If there is already a subject in the error message, the subject will be
+	// prepended to the existing subject with " > ".
+	//
+	// Subject empty string is ignored.
+	Subject(subject string) Error
+	// Subjectf is a wrapper for Subject(fmt.Sprintf(format, args...)).
+	Subjectf(format string, args ...any) Error
+}
+
+// this makes JSON marshaling work,
+// as the builtin one doesn't.
+//
+//nolint:errname
+type errStr string
+
+func (err errStr) Error() string {
+	return string(err)
+}