net/http: push roundTrip panic higher in the stack

net/http: push roundTrip panic higher in the stack

If Transport is a non-nil interface pointing to a nil implementer,
then a panic inside of roundTrip further obsfucates the issue.

Change-Id: I47664b8e6185c5f56b5e529f49022484b5ea1d94
Reviewed-on: https://go-review.googlesource.com/c/go/+/661897
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Neal Patel <nealpatel@google.com>
Reviewed-by: Damien Neil <dneil@google.com>