Fix swarming.py collect when there was no try.
Fix swarming.py collect when there was no try.

In this case, try_number is not set, which caused a crash.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1345323003

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py