running without any output specified should not pass silently

Currently if you run
`starspan --vector zonemap.shp --raster ndvi.tif --stats avg --out-prefix test --out-type table`

you might expect a zonal stats output but instead starspan exits without any stdout or output files. 

In this example, you'd need to add `--summary-suffix _stats.csv` to get zonal stats table. 

A warning should be issued in cases like this.
