Fix race and error case in GN auto-roller script.

Fix race and error case in GN auto-roller script.

There were two issues in the GN auto roller script;
in the first case, if we hit a race in Rietveld and
did not find any try jobs posted, we would return
an error code rather than an empty dict of job results.

Once that was fixed, we might still not have results
from all the bots we were waiting on, so we might exit
earlier. This patch fixes things so we wait for results
for all of the try jobs we posted.

TBR=brettw@chromium.org
BUG=

Committed: https://crrev.com/a6108a3461418a0b9928a40e712181876e2ca3f4
Cr-Commit-Position: refs/heads/master@{#349831}

[Dirk Pranke, 2015-09-19 01:00:21]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-09-19 01:00:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1357713008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1357713008/1

[commit-bot: I haz the power, 2015-09-19 01:12:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-19 01:13:06]

Patchset 1 (id:??) landed as
https://crrev.com/a6108a3461418a0b9928a40e712181876e2ca3f4
Cr-Commit-Position: refs/heads/master@{#349831}

[brettw, 2015-09-19 03:54:15]

lgtm