Improve filesystem exception messages.

BUG=270033
R=jochen@chromium.org, kinuko@chromium.org

Review URL: https://codereview.chromium.org/22825014

git-svn-id: svn://svn.chromium.org/blink/trunk@156377 bbb929c8-8fbe-4397-9dbb-9b2b20218538