Adds a notion of an (optional) error stream to the existing
log and emit streams. If not specified, the log stream is used.
Error messages in parser/translation are sent to this new error
stream.

In the browser compiler server, a separate error (string) stream is
created to capture errors. Method onEndCallBack returns the contents
of the error stream (if non-empty) instead of a generic error message.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4138
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/1052833003