more robust error handling for install2.r

[cboettig]

install2.r has long had the option -e or --error to make the script throw an error instead of the default behavior of a waring if install.packages fails to successfully install the package. Unfortunately, since R 3.4.0, utils has started throwing a warning if the target repo does not have a PACKAGES.rds in src/contrib. Many drat repositories just build src/contrib/PACKAGES and src/contrib/PACKAGES.gz, so this means that the install2.r script now fails to install from drat repos (which is very sad, because drat repos are great).

This patch provides a more robust approach to error when a package fails to install, but not simply error on any possible warning (as it does currently). This involves no change to the user interface of install2.r

[eddelbuettel]

Making this more robust is good. Medium-term we're good too because drat just calls out to the base tools to create indices which, if you use R 3.4.0 or later, will write PACKAGES.rds for you.

But that doesn't fix old repos, not does it help for older install. Will review the PR when I have moment or two and also think I should get a littler update out ...

Dirk