Better error handing for broken identify requests

[popensesame]

When the user makes an identify request, the viewer puts up a loading spinner to indicate to the user to wait. If an identify request returns a 500 error, the loading indicator remains indefinitely.

In the case of a 500 response to an identify request, seldon should remove the loading indicator, show a popup to the user that an error occurred, and return normal control to the user.