Add backtrace info to "surprise method call" logging

We are currently logging (just rails log) whenever something calls `Proposal#update_votes_count`, because Liquidvoting doesn't use it, and it will corrupt our vote count. 

The log line is nice, but doesn't help us know where it comes from; adding a backtrace in the log would do that.

This issue might be superceded by some Honeycomb events work.