It2MeHelperChannel error reporting cleanup

It2MeHelperChannel error reporting cleanup

This CL introduces a new message type - Error. All error 
message can only be reported in messages with this type. 
This will provide a consistent error reporting mechanism with It2MeHelpeeChannel

Committed: https://crrev.com/b400d928a4c4b63abbe941d02e96257e0be695f0
Cr-Commit-Position: refs/heads/master@{#293837}

[kelvinp, 2014-09-05 21:44:11]

kelvinp@chromium.org changed reviewers:
+ jamiewalch@chromium.org

[kelvinp, 2014-09-05 21:44:56]

PTAL

[Jamie, 2014-09-05 22:01:21]

LGTM with (optional) nits.

https://codereview.chromium.org/547013002/diff/1/remoting/webapp/background/i...
File remoting/webapp/background/it2me_helper_channel.js (right):

https://codereview.chromium.org/547013002/diff/1/remoting/webapp/background/i...
remoting/webapp/background/it2me_helper_channel.js:308: * @param
{?{method:string, data:Object.<string,*>}} incomingMessage
incomingMessage is never null in this code; do you anticipate that it might be
in the future? If so, consider making it an optional parameter instead; assuming
the caller always knows whether or the error is a response to a message, I think
an optional parameter would be more readable here.

https://codereview.chromium.org/547013002/diff/1/remoting/webapp/background/i...
remoting/webapp/background/it2me_helper_channel.js:316: }
I think it would be cleaner to require the caller to pass a string (or, better,
an Error, since that's what the callers are doing right now, and it would allow
you to log it with a call stack here). I'm not a fan of changing the type of a
variable, even within such a limited scope.

[kelvinp, 2014-09-08 20:04:25]

The CQ bit was checked by kelvinp@chromium.org

[commit-bot: I haz the power, 2014-09-08 20:05:23]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/kelvinp@chromium.org/547013002/20001

[commit-bot: I haz the power, 2014-09-09 02:04:54]

Committed patchset #2 (id:20001) as 4227e5d85fd916ed1257e7327cb2f4b27df2801b

[commit-bot: I haz the power, 2014-09-10 03:49:50]

Patchset 2 (id:??) landed as
https://crrev.com/b400d928a4c4b63abbe941d02e96257e0be695f0
Cr-Commit-Position: refs/heads/master@{#293837}