The fix for #51112 introduced a depth check but used
ErrInternalError to avoid introduce new API in a CL that
would be backported to earlier releases.
New API accepted in proposal #51684.
This CL adds a distinct error for this case.
For #51112.
Fixes#51684.
Change-Id: I068fc70aafe4218386a06103d9b7c847fb7ffa65
Reviewed-on: https://go-review.googlesource.com/c/go/+/384617
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
