Files.app: Avoid triggering spurious errors on cloud import cancellation.

Files.app: Avoid triggering spurious errors on cloud import cancellation.

Cancelling an import will result in abort errors for in-flight copy operations.
We don't care about these errors because they're expected, so trap and ignore
them.  This fixes a bug where the uncaught exceptions were preventing the UI
from being updated.

BUG=469855
Committed: https://crrev.com/86ffe2c8cef83afac99a842492595cd997cd100a
Cr-Commit-Position: refs/heads/master@{#321860}

[Ben Kwa, 2015-03-23 20:31:37]

kenobi@chromium.org changed reviewers:
+ smckay@chromium.org

[Steve McKay, 2015-03-23 20:59:06]

The CQ bit was checked by smckay@chromium.org

[Steve McKay, 2015-03-23 20:59:07]

lgtm

[commit-bot: I haz the power, 2015-03-23 20:59:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1016613005/1

[commit-bot: I haz the power, 2015-03-23 21:36:05]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-23 21:37:07]

Patchset 1 (id:??) landed as
https://crrev.com/86ffe2c8cef83afac99a842492595cd997cd100a
Cr-Commit-Position: refs/heads/master@{#321860}