In this change, we examine the error code returned when one of the P2P channels
is closed.  If no error is reported, then we pass OK to the error callback,
otherwise we convert to CHANNEL_CONNECTION_ERROR as before.

I am looking into the correct long-term fix for the session-terminate message
being dropped, however this change should be safer and can be merged into our
M57 release to allow us to correct the incorrect logging errors sooner.

BUG=692165

Review-Url: https://codereview.chromium.org/2708263002
Cr-Commit-Position: refs/heads/master@{#451912}