[Telemetry] Make the logging message when screenshot capturing fail less verbose.

[Telemetry] Make the logging message when screenshot capturing fail less verbose.

Adding the stack trace of screenshot failure will make it's harder for perf
sherrifs to diagnore issue. Switch this to log the exception object instead.

BUG=549326
Committed: https://crrev.com/2fa40f2cb37ef537d174e3852356ee8c257b7edf
Cr-Commit-Position: refs/heads/master@{#357012}

[nednguyen, 2015-10-29 22:42:11]

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

[aiolos (Not reviewing), 2015-10-29 23:51:50]

lgtm

[nednguyen, 2015-10-29 23:58:21]

The CQ bit was checked by nednguyen@google.com

[commit-bot: I haz the power, 2015-10-29 23:58:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1408063008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1408063008/1

[commit-bot: I haz the power, 2015-10-30 01:27:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-30 01:28:54]

Patchset 1 (id:??) landed as
https://crrev.com/2fa40f2cb37ef537d174e3852356ee8c257b7edf
Cr-Commit-Position: refs/heads/master@{#357012}