We seem to be getting some bad responses from Selenium periodically and thanks to the architecture of Behat we currently end the session in a hacky way and without giving any useful information.

We should:

1. give useful debugging information
2. only stop behat if we fail to get the session on the first attempt to get a session
3. fail a single test instead of all tests

This is a long-standing issue which has no doubt become more prominent thanks to MDL-69072 which restarts the session between every test.