Give up after certain number requests that return no data

[rweigel]

The following continues to run for a long time because requests return no data.

https://hapi-server.org/verify/?url=https://iswa.gsfc.nasa.gov/IswaSystemWebApp/hapi&id=NewHorizonsFlyBy_USMANOVatNewHorizons&parameter=NumberDensity&time.min=2015-10-31T15:37:00Z&time.max=2015-10-31T15:52:00Z