Add path to error message in pepper mount.
Add path to error message in pepper mount.

BUG=None
TEST=None
R=jeffbailey@google.com
Review URL: https://chromiumcodereview.appspot.com/10880035

git-svn-id: 84b3587ce119b778414dcb50e18b7479aacf4f1d@630 7dad1e8b-422e-d2af-fbf5-8013b78bd812

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