Show error if wrong password is specified for a display device in UI

When user wants to access a VM over a display device, he/she is presented with an option to select which display device they would like to use. It uses the endpoint "vm.get_display_web_uri". The endpoint has been updated to return error string as well for a display device if an error is generated when retrieving the URI for a display device.

An example for an error can be user specifies wrong password, in this case system returns the appropriate response and we should present this to the user.

{"196": {"error": null, "uri": "http://192.168.0.3:5803/spice_auto.html"}}

196 is VM device id. Please let me know if there are any queries, thank you.