os: have UserCacheDir return an error on failure

os: have UserCacheDir return an error on failure

Previously, it would return an empty string if it
could not determine the user's cache directory.
Return an error instead.

Change-Id: I74f00b1ad3858efa3fe2700c599271ebfe5764b6
Reviewed-on: https://go-review.googlesource.com/120757
Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>