transport: make http2Client.getStream() only return a stream.

[easwars]

Suggested in #2980.

[iwanbk]

hi @dfawley @easwars,
as far as i can see, this change doesn't add any added value, the caller of getStream also doesn't get shorter. On the other hand:

• it is not idiomatic, see https://github.com/golang/go/wiki/CodeReviewComments#in-band-errors

Go's support for multiple return values provides a better solution. Instead of requiring clients to check for an in-band error value, a function should return an additional value to indicate whether its other return values are valid.

This prevents the caller from using the result incorrectly:

• it becomes error-prone, as the caller might forget to do if err == nil check, which is not common things to do.