Fix installation of unapproved plugins failing

More `errexit` malarkey: with it set (aka `set -e`, which *rbenv* does), checking or assigning `$?` is an anti-pattern. In the case of `rbenv-plugin-install`, it will lead to the install process quietly failing when the `grep` run on the “approved” plugins file returns no match. This PR re-instates the ability to install “unapproved” plugins.