quic: add Stream.StopSending, and rework stream/application errors

Rename StreamErrorCode to StreamError, to maintain the convention that
error type names end with "Error".

Rename ApplicationError to ConnectionCloseError, to distinguish it
from StreamError.

Add a Stream.StopSending method, which is Stream.CloseRead but permits
setting the application error code to send.

Add an error return value to Stream.CloseRead and Stream.CloseWrite,
to match the signature of net package methods with these names.

Change-Id: Ie84b4b0b874636deae8adc9de26e90886a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/825004
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>