Comment 2 for bug 1421754

http://helioviewer.org/?date=2013-02-09T18:13:15.000Z&imageScale=154.90822&centerX=0&centerY=0&imageLayers=%5BSTEREO_A,SECCHI,COR2,white-light,0,100%5D&eventLayers=&eventLabels=true

   {"observatory":"STEREO_A","instrument":"SECCHI","detector":"COR2","measurement":"white-light","name":"COR2-A","sourceId":29,"layeringOrder":0,"visible":false,"opacity":100}
   PHP Fatal error: Uncaught exception 'Exception' with message 'No valid and visible layers specified for request.'

   Page fails to load due to exception when layering order is set to zero and there are no other visible layers in the request.

http://helioviewer.org/?date=2013-02-09T18:13:15.000Z&imageScale=154.90822&centerX=0&centerY=0&imageLayers=%5BSTEREO_A,SECCHI,COR2,white-light,1,100%5D&eventLayers=&eventLabels=true

   {"observatory":"STEREO_A","instrument":"SECCHI","detector":"COR2","measurement":"white-light","name":"COR2-A","sourceId":29,"layeringOrder":1,"visible":true,"opacity":100}

   Page loads normally when setting layeringOrder > 0