Adjust exception logging for non-critical - or potentially spammy - situations

[klauer]

We can do better as to the user-friendliness of most of our log messages.

For example, if beacon broadcasts are failing, 10 lines per failed send with an exception traceback is excessive and not helpful to the end user.

Some thoughts:

• Log level adjustment: debug has full traceback, such that -vv can be used to provide bug reports
• warning can be used for user-friendly messages, sans-traceback
• Truly critical errors - to the best of our knowledge - should probably still have error set with exc_info (or .exception())

(Half-baked thought at this point; will come back with further details when I have the time. Input/feedback very much welcome.)

[danielballan]

For beacons in particular, if they are repeatedly failing to send it might makes sense to warn one (last) time and keep quiet, or warn only occasionally.