Fix GOAWAY errors properly

[MikeBishop]

In #2705, @DaanDeMeyer correctly observed that I messed up #2343, but he closed his PR and I'm not able to reopen it without more git wizardry than I'm interested in practicing today. Easier just to fix it in my own branch.

So:

• GOAWAY properly sent by the server on the control stream, but containing the wrong stream ID can't be WRONG_STREAM, it should be MALFORMED_FRAME.
• GOAWAY sent by the server on something other than the control frame isn't UNEXPECTED_FRAME, it's WRONG_STREAM (see aside with @kazuho about whether the distinction actually matters, but in the interest of keeping the document internally consistent....)
• GOAWAY sent by a client is UNEXPECTED_FRAME, and that's the only one I got right before.

Fixes #2714.

[MikeBishop]

Whoops -- missed that #2707 was the updated version of #2705. Closing in favor of @DaanDeMeyer's, which contains identical text.