improve version error formatting

error.go

@@ -0,0 +1,37 @@
+package version
+
+import "fmt"
+
+// Error represents a version error.
+type Error struct {
+	Message string
+	Version string
+}
+
+// Version error.
+var (
+	ErrInvalidVersion = Error{Message: "invalid version %q"}
+)
+
+// Error retrieves the message of a REST API error.
+func (e Error) Error() string {
+	return fmt.Sprintf(e.Message, e.Version)
+}
+
+// Is determines whether the Error is an instance of the target.
+// https://pkg.go.dev/errors#Is
+//
+// This implementation does not compare versions.
+func (e Error) Is(target error) bool {
+	if t, ok := target.(Error); ok {
+		return t.Message == e.Message
+	}
+	return false
+}
+
+func invalid(version string) Error {
+	return Error{
+		Message: ErrInvalidVersion.Message,
+		Version: version,
+	}
+}