path/filepath: return special error from EvalSymlinks

CL 155597 attempted to fix #29372. But it failed to make all new
test cases pass. Also CL 155597 broke some existing code
(see #29449 for details).

Make small adjustment to CL 155597 that fixes both #29372 and #29449.

Suggested by Ian.

Updates #29372
Fixes #29449

Change-Id: I9777a615514d3f152af5acb65fb1239e696607b6
Reviewed-on: https://go-review.googlesource.com/c/156398


Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>