Summary: Clean handling of lack of $EDITOR.
Problem: If EDITOR is not set, and vim is not installed, rbt post exits with just "CRITICAL: [Errno 2] No such file or directory" as an error message, which does not indicate where the problem lies nor how to solve it.

Discussion: vim is not installed by default on many distributions, or is installed as "vi". Moreover, EDITOR is traditionally used for a line editor (remember those?), with VISUAL being the environment variable of choice for a full-screen or GUI editor. If neither is set, we should fall back to the editor most likely to be installed in the base distribution.
Solution: Prefer $VISUAL over $EDITOR over "vi". Offer a helpful error message on failure.

yes.