path/filepath: add explicit comment marking elided errors

The errors on these lines are meant to be discarded.
Add a comment to make that extra clear.

Change-Id: I38f72af6dfbb0e86677087baf47780b3cc6e7d40
Reviewed-on: https://go-review.googlesource.com/c/go/+/241083


Reviewed-by: Ian Lance Taylor <iant@golang.org>