Import some choreographer stuff into kaleido for convenience

[ayjayt]

From @emilykl

There’s a few classes/functions that would be nice to be able to import from Kaleido directly rather than Choreographer — I’d like access to them for the Plotly.py integration but don’t really want to import choreographer :
    choreographer.ChromeNotFoundError — so that we can catch the error on figure generation and display an error message that comes from Plotly rather than Kaleido
    choreographer.cli._cli_utils.get_chrome_download_path() — so that we can display the installation path to the user before performing the Chrome install. In addition that function seems like an internal function, would nice to make it part of the official API.