DevTools: protocol error responses to return error details.

This patch adds ExceptionState::message() method to return exception
message. The method is used to generate protocol error responses.

BUG=

Review URL: https://codereview.chromium.org/190673002

git-svn-id: svn://svn.chromium.org/blink/trunk@168919 bbb929c8-8fbe-4397-9dbb-9b2b20218538