Add error checking to R package installation

If an error occurred installing an R package, the Ansible command still
suceeded. Adding a final import of the package cause Ansible to notify us
if anything went wrong.
Ensure devtools gets installed correctly

devtools depends on R package RCurl which depends on command curl-config
which comes from the libcurl4-gnutls-dev library.

Separate the original setup from the new devtools/popdemo setup, in case
popdemo becomes supported again, and we want to revert