Markdown returns a wrapped error

renderers/markdown.go

@@ -2,10 +2,10 @@ package renderers
 
 import (
 	"bytes"
-	"fmt"
 	"io"
 	"regexp"
 
+	"github.com/osteele/gojekyll/utils"
 	"github.com/russross/blackfriday"
 	"golang.org/x/net/html"
 )
@@ -39,7 +39,7 @@ func renderMarkdown(md []byte) ([]byte, error) {
 	)
 	html, err := renderInnerMarkdown(html)
 	if err != nil {
-		return nil, fmt.Errorf("%s while rendering markdown", err)
+		return nil, utils.WrapError(err, "markdown")
 	}
 	return html, nil
 }

utils/errors.go

@@ -5,12 +5,40 @@ import (
 	"os"
 )
 
+// A WrappedError decorates an error with a message
+type WrappedError interface {
+	error
+	Cause() error
+}
+
+// WrapError returns an error decorated with a message.
+// If the error is nil, it returns nil.
+func WrapError(err error, m string) error {
+	if err == nil {
+		return nil
+	}
+	return &wrappedError{cause: err, message: m}
+}
+
+type wrappedError struct {
+	cause   error
+	message string
+}
+
+func (we *wrappedError) Cause() error {
+	return we.cause
+}
+
+func (we *wrappedError) Error() string {
+	return fmt.Sprintf("%s: %s", we.message, we.cause)
+}
+
 // A PathError is an error with a source path.
 //
 // An os.PathError is unfortunately not a PathError, but this is still
 // useful for deciding whether to wrap other errors.
 type PathError interface {
-	error
+	WrappedError
 	Path() string
 }
 
@@ -19,6 +47,10 @@ type pathError struct {
 	path  string
 }
 
+func (pe *pathError) Cause() error {
+	return pe.cause
+}
+
 func (pe *pathError) Error() string {
 	return fmt.Sprintf("%s: %s", pe.path, pe.cause)
 }