ogr plugin extent error handling

Added exception in OGR plugin for case when OGR layer does not have an extent and no custom extent is provided via the <extent> parameter.  
Here was my line of thinking - [OGR docs](http://www.gdal.org/classOGRLayer.html#a3be658ddb5b33d1ed95c31286774bbd2)  says OGRERR_FAILURE is returned "if extent not known".  I didn't want to trap for other return values that may get implemented later and possibly mask bugs.    

I'll let you make the final call