Return diagnostic information from checkpoint() #163
Description
Currently, the checkpoint() function always returns NULL.
It would be useful to (invisibly) return more information, including lists of:
- packages discovered
- discovered packages that don't exist in the MRAN snapshot
- scanned R scripts that contained parse errors (and thus required packages could not be detected)
- packages that failed to install correctly (e.g. because of missing system dependencies)
Use case: This allows the user to more easily (programmatically) fix detected errors. Right now, the only way to find this information is in the printed messages, which might scroll out of view.