Implemented function scanning

src/errors/InvalidTopLevel.go

@@ -0,0 +1,13 @@
+package errors
+
+import "fmt"
+
+// InvalidTopLevel error is created when a top-level instruction is not valid.
+type InvalidTopLevel struct {
+	Instruction string
+}
+
+// Error implements the error interface.
+func (err *InvalidTopLevel) Error() string {
+	return fmt.Sprintf("Invalid top level instruction '%s'", err.Instruction)
+}