Telemetry: Finish documenting exceptions thrown by tab.py.

Telemetry: Finish documenting exceptions thrown by tab.py.

All the major work has already been done. This CL just finishes flushing out
the documentation on tab.py and web_contents.py so that consumers know exactly
which exceptions can be thrown.

This CL contains one small behavioral change: InspectorBackend.IsInspectable()
catches DevToolsClientConnectionError and returns False. This is useful since
IsInspectable() gets called in the exception handling code, and no other
consumers of IsInspectable expect the method to throw an exception.

BUG=460625
Committed: https://crrev.com/4596fc4c841d807fe165fc8fcd5cb0f1fdb89b98
Cr-Commit-Position: refs/heads/master@{#321393}

[erikchen, 2015-03-18 23:00:12]

erikchen@chromium.org changed reviewers:
+ dtu@chromium.org

[erikchen, 2015-03-18 23:00:12]

dtu: Please review.

[dtu, 2015-03-18 23:01:19]

lgtm

[erikchen, 2015-03-18 23:01:43]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-03-18 23:02:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1007903004/1

[commit-bot: I haz the power, 2015-03-19 00:13:59]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-03-19 00:14:00]

Try jobs failed on following builders:
  linux_chromium_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[erikchen, 2015-03-19 00:20:28]

The CQ bit was checked by erikchen@chromium.org

[erikchen, 2015-03-19 00:20:29]

The patchset sent to the CQ was uploaded after l-g-t-m from dtu@chromium.org
Link to the patchset: https://codereview.chromium.org/1007903004/#ps20001
(title: "Rebase against top of tree.")

[commit-bot: I haz the power, 2015-03-19 00:21:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1007903004/20001

[commit-bot: I haz the power, 2015-03-19 02:06:34]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-03-19 02:06:35]

Try jobs failed on following builders:
  linux_chromium_asan_rel on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[erikchen, 2015-03-19 17:43:17]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-03-19 17:44:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1007903004/20001

[commit-bot: I haz the power, 2015-03-19 18:22:48]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-19 18:23:41]

Patchset 2 (id:??) landed as
https://crrev.com/4596fc4c841d807fe165fc8fcd5cb0f1fdb89b98
Cr-Commit-Position: refs/heads/master@{#321393}