improve robustness to untar2/incomplete block on file errors

[bbolker]

I realize this is a niche problem, but ... I'm trying to do a large revdep check (500+ packages) on a machine with a flaky network connection, and am frequently running into transient errors

.Error in untar2(tarfile, files, list, exdir, restore_times) : incomplete block on file

I currently have my revdep_check() call wrapped in a repeat { r <- try(...); if (!inherits(r, "try-error")) break } block, which helps a bit, but not enough — if a download fails before any of the workers has succeeded in fully checking a package, then that iteration of the loop doesn't make any progress.

I am willing to take a shot at hacking this myself but wouldn't mind opinions/input - in particular, I would like to implement (1) a few retries at downloading the package (seems easy enough) and (2) giving up and skipping further attempts (in the current revdep_check() invocation) to check the package that has failed to download.