Add an explicit way for Handlers to abort their response to the client
and also not spam their error log with stack traces.

panic(nil) also worked in the past (for http1 at least), so continue
to make that work (and test it). But ErrAbortHandler is more explicit.

Updates #17790 (needs http2 updates also)

Change-Id: Ib1456905b27e2ae8cf04c0983dc73e314a4a751e
Reviewed-on: https://go-review.googlesource.com/33099
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>