Add new error messages to socket.cc in forwarder2.

TBR=bulach@chromium.org
BUG=345273

Review URL: https://codereview.chromium.org/179003004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253161 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253161