Make it clear that server plugins are optional

[nirvdrum]

After I pulled from master and started up the server, I was greeted with the message "Cannot import yapsy; server plugin functionality not available.". I assumed this was a new requirement and something I needed to have running, rather than something that was optional. A softer worded message, that indicates the functionality is optional, would probably be better.

[calmofthestorm]

#50 should address this. Thanks!