How to inform Travis CI to expect an error and don't fail?

I have an example-repo that shows the usage of a package.

To show error propagation, code is written in a way that it's expected to print an error. It writes a few lines to stderr which are considered as an error by Travis CI and tests fail. How can I tell Travis CI to expect an error from this line and don't fail?

PS: I don't want to write tests and define there to expect an error.