Rescue and log on Process.setpgrp exception.

Hey,

Really excited about leveraging foreman!  I'm learning more about it, but ran into this issue.  

Right now, I'm running ```foreman start``` from within a systemd unit.  Since systemd is already managing the process group, foreman gets a permissions error when trying to set the process group.

I'm starting to think that I may need to actually find/write a systemd exporter, and then manage the individual processes with systemd rather than foreman (so foreman would be used exclusively for exporting).  That approach has some nice things about it, but I think my naive use-case is still a viable one.

It's a quick fix to swallow the Process.setpgrp permissions error.  Do you this this approach is valid or am I looking at it backwards and should write an exporter?  Thanks!

-Nick