pkcs12: fix ToPEM to not ignore non-nil error from getSafeContents
pkcs12: fix ToPEM to not ignore non-nil error from getSafeContents

Fixes golang/go#19542

Change-Id: I60d2370d1d1e0763c72c9cc203ea2ff21123af73
Reviewed-on: https://go-review.googlesource.com/38261
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/crypto,ScriptRock/crypto,ScriptRock/crypto,golang/crypto,golang/crypto