The new authorization flow also requires the user to input a code
returned on the browser's authorization page.
Since the Google authentication libraries seem to use a simple `input()`
to read this code, things are likely to fail quite badly if Platypush is
run in non-interactive mode - e.g. through a systemd service or in a
Docker container.
Thus we need to go with Google's automatic flow only if `DISPLAY` or
`BROWSER` are set (thus the interactive flow can proceed), and throw an
`AssertionError` with the command to execute if no display/browser are
detected.
Expanded explanation of the desktop vs. headless authentication process
and where the credential files are supposed to be copied if the user
opts for a non-automatic authentication flow.
Fabio Manganiello