Save Twilio errors information

[spalladino]

Any errors when issuing a Twilio outgoing call (such as an invalid number) are now captured and displayed in the call log details, along with a link to the Twilio documentation for more information.

Fixes #742

[ggiraldez]

I think it's probably better to use xmerl_xs here. See http://erlang.org/doc/man/xmerl_xs.html

Something like:

parse_exception(Reason, Body) ->
  try
    {Doc, []} = xmerl_scan:string(Body),
    [Exception] = xmerl_xs:select("/TwilioResponse/RestException", Doc),
    FullCode = case xmerl_xs:value_of(xmerl_xs:select("./Code", Exception)) of
      [Code] -> "twilio" ++ Code;
      _ -> undefined
    end,
    [Message] = xmerl_xs:value_of(xmerl_xs:select("./Message", Exception)),
    {error, Message, FullCode}
  catch
    _ -> Reason
  end.