Fix bug in UnknownAuthorityError.Error that would never allow Org
Name to be inserted into error message if the Common Name was empty.
Create tests for all three paths in UnknownAuthorityError.Error

Change-Id: Id8afc444e897ef549df682d93a8563fd9de22a2b
Reviewed-on: https://go-review.googlesource.com/27992
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>