`download_release_group_cover_art_front` is exactly one character less than

`get_release_group_cover_art_front_image`. (written in the next line just to show the "difference)

About having long names in general:
We could introduce a new `musicbrainzngs.coverart` or (`musicbrainzngs.ca`) namespace. This way we don't have to make `cover_art` part of the function name, which is the real problem I'd say. (would lead to `get_release_group_front_image`)
(the first two `get_` functions might still need some indication that we are talking about cover art lists)