Improve sign-in error UX.

Improve sign-in error UX.

If the user cancels their sign-in, or if some other error occurs, we
currently display "An unknown error occurred", which is not very helpful.

Committed: https://crrev.com/7c4333a67594fc58d9a88caab055c96e9064597a
Cr-Commit-Position: refs/heads/master@{#295800}

[Jamie, 2014-09-19 21:33:44]

jamiewalch@chromium.org changed reviewers:
+ garykac@chromium.org

[Jamie, 2014-09-19 21:33:44]

ptal

[garykac1, 2014-09-19 21:35:30]

garykac@google.com changed reviewers:
+ garykac@google.com

[garykac1, 2014-09-19 21:35:30]

lgtm

[Jamie, 2014-09-19 21:39:50]

The CQ bit was checked by jamiewalch@chromium.org

[commit-bot: I haz the power, 2014-09-19 21:41:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/585913003/1

[commit-bot: I haz the power, 2014-09-19 21:41:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/585913003/1

[commit-bot: I haz the power, 2014-09-19 23:18:17]

Committed patchset #1 (id:1) as d463f6b84e8a56d44904ec1418c99d7b47ea26dc

[commit-bot: I haz the power, 2014-09-19 23:19:18]

Patchset 1 (id:??) landed as
https://crrev.com/7c4333a67594fc58d9a88caab055c96e9064597a
Cr-Commit-Position: refs/heads/master@{#295800}