The unexported field is hidden from reflect based marshalers, which
would break otherwise. Also, make it return an error, as there are
multiple reasons it might fail.

Fixes #27125

Change-Id: I92adade2fe456103d2d5c0315629ca0256953764
Reviewed-on: https://go-review.googlesource.com/130535
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>