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This change adds more descriptive mount errors in order to display more
detailed reasons for a failed mount to the user.

Bug: chromium:757625
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I11d46ca74d40d77276bce6839899ac723b3b77c6
Reviewed-on: https://chromium-review.googlesource.com/1060428
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559395}