No clear error/alert is raised if the server returns a code 500 to a query

[153957]

In jSparc analysis session if you try to get an example the server returns an 500 error (at this moment, because there are no sessions still active), this is not made clear to the user, and the shower animation just keeps running.

[153957]

This particular error is probably easily temporarily remedied by creating a new session with >4000 coincidences and an end date of 2038. Also a proper error_json is now returned when no coincidences are available for example sessions.