Use different errors for connection closed between frames and within a frame

To aid debugging, the errors retuned by the connection should distinguish between the peer closing the connection within a frame and between frames. Closing the connection without a close frame is abnormal, but the RFC does allow for that.

The error websocket.errUnexpectedEOF is retuned for both cases.

The two cases are easily distinguished by peeking for one byte at the beginning of advanceFrame.

There are two ways to distinguish the error:

• Return websocket.errUnexpectedEOF for the expected case and io.ErrUnexpectedEOF for the unexpected case.
• Create a new 1006 close error with status text "EOF" instead of "unexpected EOF".

A somewhat related issue is adding support for polling the connection with read timeouts. This feature was requested more than once in the issues.

The advanceFrame function can be written to peek all of the bytes in the frame header and discard the bytes from the bufio.Reader after successfully reading the frame header.

If a timeout occurs when peeking the bytes, then advanceFrame should return a temporary error. If io.EOF is detected when peeking the first byte, then the "EOF" error should be returned. If io.EOF is detected on later bytes, then the "unexpected EOF" error should be retuned.

[garyburd]

Changing the text to help debug is OK.

The suggestion to handle read timeouts in advanceFrame is a good one, but a change this complex should wait until a a new maintainer is found. I don't want to review it.