Added more tests

src/build/errors/UnknownFunction.go

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