No error when R package install fails.

[rmnppt]

This is very much related to #24 but I am getting errors in loading R packages. If the install of a package fails at build, should this not break the build process with the error propagated from R. Or else is there a build log that can be inspected by the user?

[jamesatha]

When packages fail to build, R seems to print warning messages but silently keeps going. We plan to make this better soon. Thanks for bringing this to our attention