Merge pull request sirupsen#901 from CodeLingoBot/rewrite

Fix error formatting based on best practices from Code Review Comments

json_formatter.go

@@ -114,7 +114,7 @@ func (f *JSONFormatter) Format(entry *Entry) ([]byte, error) {
 		encoder.SetIndent("", "  ")
 	}
 	if err := encoder.Encode(data); err != nil {
-		return nil, fmt.Errorf("Failed to marshal fields to JSON, %v", err)
+		return nil, fmt.Errorf("failed to marshal fields to JSON, %v", err)
 	}
 
 	return b.Bytes(), nil