Missing error handling example in README

[dbackeus]

We spent quite some time trying to figure out error handling from the Lokka clients point of view.

In the end we realized that the graphql errors are assigned to rawError on the exception object by studying the code.

This debugging experience could be made better with better docs. Best would be to actually cover this in the Lokka README but since the code belongs in the transport right now maybe you would like to opt for having it in this repo...