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,
+	}
+}

errors.go

@@ -1,7 +0,0 @@
-package version
-
-import "errors"
-
-var (
-	ErrInvalidVersion = errors.New("invalid version")
-)

parse.go

@@ -24,7 +24,7 @@ func Parse(str string) (*Version, error) {
 	v := &Version{Text: string(str)}
 
 	if len(str) == 0 {
-		return nil, ErrInvalidVersion
+		return nil, invalid(str)
 	}
 
 	section := sectionMajor
@@ -32,13 +32,13 @@ func Parse(str string) (*Version, error) {
 
 	commit := func() error {
 		if len(chars) == 0 {
-			return ErrInvalidVersion
+			return invalid(str)
 		}
 
 		if section < sectionExtension {
 			n, err := strconv.Atoi(string(chars))
 			if err != nil {
-				return ErrInvalidVersion
+				return invalid(str)
 			}
 
 			switch section {

parse_test.go

@@ -35,14 +35,14 @@ func TestVersion_Parse(t *testing.T) {
 
 		if testCase.Err != nil {
 			if err == nil {
-				t.Errorf("test %d failed (expected error %v, actual nil)", i, testCase.Err)
+				t.Errorf("test %d failed (expected error %s, actual nil)", i, testCase.Err)
 			} else if !errors.Is(err, testCase.Err) {
-				t.Errorf("test %d failed (expected error %q, actual error %q)", i, testCase.Err, err)
+				t.Errorf("test %d failed (expected error %s, actual error %s)", i, testCase.Err, err)
 			} else {
-				t.Logf("test %d passed with error %q for %q\n", i, testCase.Err, testCase.Input)
+				t.Logf("test %d passed with error %s for %q\n", i, err, testCase.Input)
 			}
 		} else if err != nil {
-			t.Errorf("test %d failed (expected error nil, actual error %q)", i, err)
+			t.Errorf("test %d failed (expected error nil, actual error %s)", i, err)
 		} else if actual.Major != testCase.Expected.Major || actual.Minor != testCase.Expected.Minor || actual.Patch != testCase.Expected.Patch || actual.Text != testCase.Expected.Text {
 			t.Errorf("test %d failed (expected %v, actual %v)", i, testCase.Expected, actual)
 		} else {