http2: mention bogus header value in error messages

Change-Id: I69d713341b878a2343bb85fb2de2ef49441fd16d
Reviewed-on: https://go-review.googlesource.com/30098
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed