SPICE-HTML5: Improve error message

Current error message in SPICE-HTML5 error is poor (error-logging function
prints error.toString() which doesn't contain anything detailed).

This patch enhances this message in the following way:
 - error event contains target attribute ([1]),
 - the code in this patch checks for presence of 'url' attribute (which is
   contained in WebSocket object[2]). If the target contains url then
   meaningful message is logged.

[1]: https://developer.mozilla.org/en-US/docs/Web/API/Event
[2]: https://developer.mozilla.org/en-US/docs/Web/API/WebSocket