exception handling in _get_flag and documentation update

In this PR I fixed the documentation to use `False` rather than `false` and also added exception handling to try to explain what's going on if you don't receive a 2xx response.  Earlier today when I created an account for our dev environment I had neglected to set up a new Feature on the launch darkly admin screen, and got the "Unhandled exception in get_flag" error message
I think we still want to return `default` if `_evaluate` returns `None`, right?