GN: Make file/dir resolving return errors.

Previously GN would silently eat some types of errors when resolving files or directory names, leading to unexpected behavior.

This adds error checking to the resolution functions.

Review URL: https://codereview.chromium.org/1155713006

Cr-Original-Commit-Position: refs/heads/master@{#332697}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ff0986feacdae709a944b7cc6ee990b6cc304b76