Add counter and logging of protocol errors

[gavin-norman-sociomantic]

A protocol error that occurs while handling a request represents a serious error. These should not go unnoticed. If we write them to the stats log, then we can set up alerting in the live system to notify maintainers.

[gavin-norman-sociomantic]

The ProtocolError exception is used for several different things in different modules, for example:

1. Connection-level (e.g. establishment / authentication) errors.
2. Message-level errors.
3. Request-level protocol errors, in request handlers.

Each type of error could be counted separately, as they represent different things, really.

[gavin-norman-sociomantic]

In addition to the different types of protocol error mentioned above, it'd also be nice to catch and log arbitrary errors from other sources (e.g. verify calls throwing).

[gavin-norman-sociomantic]

Logging of auth errors added in #379. (I'm not 100% sure that covers auth protocol errors, though.)