parsePrivateKey can't return useful error messages because it does trial
decoding of multiple formats.  Try ParseCertificate first in case it
offers a useful error message.

Fixes #23591

Change-Id: I380490a5850bee593a7d2f584a27b2a14153d768
Reviewed-on: https://go-review.googlesource.com/90435
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>