Reject overlarge frames with RST_STREAM(FRAME_SIZE_ERROR), not GOAWAY(PROTOCOL_ERROR)

[Lukasa]

When receiving overlarge frames we currently tear the whole connection down as a PROTOCOL_ERROR. We should either do that but signal the real problem (FRAME_SIZE_ERROR) or, even better, send RST_STREAM instead, per Section 4.2 of RFC 7540.

Found by h2spec:

  4.2. Frame Size
    × Sends large size frame that exceeds the SETTINGS_MAX_FRAME_SIZE
      - The endpoint MUST send a FRAME_SIZE_ERROR error.
        Expected: GOAWAY frame (ErrorCode: FRAME_SIZE_ERROR)
                  RST_STREAM frame (ErrorCode: FRAME_SIZE_ERROR)
                  Connection close
          Actual: GOAWAY frame (Length: 8, Flags: 0, ErrorCode: PROTOCOL_ERROR)

[Lukasa]

Frankly, I'm disinclined to interoperate with an implementation that is sending oversized frames after a SETTINGS ACK, so let's tear the whole connection down.