Server-side functions run in the puppetserver JRuby, whose gem path is
separate from the agent CRuby one, so catalog compiles fail with
`LoadError: no such file to load -- toml`.
- Install toml via `puppetserver gem`, matching the puppetserver_gem
resource in puppet-prod's profiles::puppet::gems
- Report each failed gem install to stderr instead of exiting silently
- Keep the hook exiting zero: the entrypoint runs post-startup hooks
under errexit, where a non-zero exit kills a serving compiler
unkin-agent