tape: Add limits to the API

tape/error.go

@@ -0,0 +1,11 @@
+package tape
+
+// Error enumerates common errors in this package.
+type Error string; const (
+	ErrTooLong Error = "data structure too long"
+)
+
+// Error implements the error interface.
+func (err Error) Error() string {
+	return string(err)
+}