Add unit tests for warning/error messages

## Description
Error and warning messages were fixed in [ppp-web issue 19](https://github.com/PMA-2020/ppp-web/issues/19).

We should add unit tests for warning/error messages.

[error and warning unit tests.zip](https://github.com/PMA-2020/ppp-web/files/2479287/error.and.warning.unit.tests.zip)