[Telemetry] Do not close tab connections if the tab is already gone.

[Telemetry] Do not close tab connections if the tab is already gone.
Update tests so they check for formatted exceptions (so they will catch
a bug such as this one).

BUG=444240
Committed: https://crrev.com/86133f6e51b06f58afdcf4b00e65ffb3f510f34b
Cr-Commit-Position: refs/heads/master@{#322613}

[slamm, 2015-03-27 17:00:01]

slamm@chromium.org changed reviewers:
+ nednguyen@google.com

[slamm, 2015-03-27 17:04:26]

comment tweak

[nednguyen, 2015-03-27 17:21:59]

lgtm

https://codereview.chromium.org/1041843002/diff/20001/tools/telemetry/telemet...
File tools/telemetry/telemetry/page/page_run_end_to_end_unittest.py (right):

https://codereview.chromium.org/1041843002/diff/20001/tools/telemetry/telemet...
tools/telemetry/telemetry/page/page_run_end_to_end_unittest.py:108: return
self._formatted_exception_buffer.getvalue()
So cool!

[slamm, 2015-03-27 17:23:37]

The CQ bit was checked by slamm@chromium.org

[commit-bot: I haz the power, 2015-03-27 17:24:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1041843002/20001

[commit-bot: I haz the power, 2015-03-27 18:45:26]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-27 18:46:20]

Patchset 2 (id:??) landed as
https://crrev.com/86133f6e51b06f58afdcf4b00e65ffb3f510f34b
Cr-Commit-Position: refs/heads/master@{#322613}

[slamm, 2015-03-30 04:00:04]

A revert of this CL (patchset #2 id:20001) has been created in
https://codereview.chromium.org/1047613002/ by slamm@chromium.org.

The reason for reverting is: https://crbug.com/471512
Flaky telemetry_perf_unittests on all platforms due to ChromeTracingStartedError

Setting current_tab to None should be done for the crash tab/app errors only.

Also, consider guarding the other clean-up methods that use it, or make sure
they can dean with a None value..