Good points, but I think they'll see an error in either case, since `rbundler` will install the package itself as a last step: https://github.com/opower/rbundler/blob/master/R/bundle.r#L68

I tend to think that en error up-front is better than a downstream error. That said, if we take your warning approach, then it might make sense to _not_ install the package itself in the last step. Removing package installation as a side-effect will give the caller (or user) more control over workflows. For example, this would allow someone to use devtools to simply reload the scripts, as opposed to installing the package. I'd love to get your opinion on this.