Refactored code structure

src/build/errors/UnknownIdentifier.go

@@ -0,0 +1,18 @@
+package errors
+
+import "fmt"
+
+// UnknownIdentifier represents unknown variables.
+type UnknownIdentifier struct {
+	Name        string
+	CorrectName string
+}
+
+// Error generates the string representation.
+func (err *UnknownIdentifier) Error() string {
+	if err.CorrectName != "" {
+		return fmt.Sprintf("Unknown identifier '%s', did you mean '%s'?", err.Name, err.CorrectName)
+	}
+
+	return fmt.Sprintf("Unknown identifier '%s'", err.Name)
+}