Allow customizing http and websocket status codes for errors

[vektah]

This PR sets a general rule that:

• parse and validation errors are 422s
• resolver errors are 200s
• anything else is a 200.

same applies to websockets, except they use GQL_ERROR and GQL_DATA message types instead of status codes.

But we need a way to distinguish between the types of errors. Apollo sets a "code" extension, lets copy them and match their codes. I've included the two core ones in the errcode package, you can return anything and they will be treated as 200s by default. To register a custom 422:

func init()  {
   errcode.Register("MY_CODE", errcode.KindProtocol) // make this code a 422 
}
// when you return an error:
errcode.Set(err, "MY_CODE")

I've also added integration tests for websockets and fixes a bunch of websocket related issues, mostly around error handling.

closes #948
closes #965

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[vvakame]

Hi! I'm back from holiday today. sorry for late response 😉
I think this PR is nice than my PR 👍