Motivation:

Receiving a CONTINUATION frame at the wrong stage of a connection is
a PROTOCOL_ERROR, but out of an abundance of caution we were categorising
it as an INTERNAL_ERROR. We should be more confident about our error
categories.

Modifications:

- Changed INTERNAL_ERROR to PROTOCOL_ERROR.
- Added a test for this error case.

Result:

More confidence, more style, more pizazz.