services should be allowed to start with a warning if icon not provided or does not exist

[piyushk]

Mostly an enhancement. I don't want to copy placeholder icons around. I got the following error:

could not find service profile[concert_service_gazebo/segbot]

generated from https://github.com/robotics-in-concert/rocon_concert/blob/hydro-devel/concert_service_manager/src/concert_service_manager/service_profile.py#L73
because my icon did not exist - https://github.com/robotics-in-concert/rocon_concert/blob/hydro-devel/concert_service_manager/src/concert_service_manager/service_profile.py#L141

[stonier]

Could definitely at least use a better feedback message.

I think I prefer stopping the service though. It helps the user get their service profile factually correct, i.e. in this case deleting the icon parameter is sufficient for catering to those who haven't gotten around to configuring an icon and also makes sure the service profile doesn't present information which actually isn't true.