Bridges with exit flag: when running a bridge with an exit policy, Tor should warn the operator that won't work as exit

Some operators believed that it was possible to run both an exit and a bridge ('multi-purpose' relay) using the same Tor process and torrc configuration. They were surprised that tor didn't fail to start with that configuration. See: tpo/network-health/team#289 (closed)

To address this, maybe we could add a warning to the tor logs, such as:

"Warn: You are running a bridge, but it appears that you have also configured an exit policy. Tor will start, but it will not function as an exit relay."

What do you think?