added test to check whether FeatureSet.OpenFile returns the expected exception when a file is missing (#996)
added test to check whether FeatureSet.OpenFile returns the expected exception when a file is missing (#996)

DotSpatial/DotSpatial,pergerch/DotSpatial,DotSpatial/DotSpatial,pergerch/DotSpatial,CGX-GROUP/DotSpatial,CGX-GROUP/DotSpatial,pergerch/DotSpatial,CGX-GROUP/DotSpatial,DotSpatial/DotSpatial