Resolve "make error messages more descriptive"

Closes #8

gridded_glm/libexec/_minute_gridder.py

@@ -157,6 +157,10 @@ def grid_setup(args, work_dir=os.getcwd()):
             log.error("Tried to grid file that does not exist: {}".format(f))
             exit(1)
 
+    if args.goes_sector == "meso" and (args.ctr_lat == None or args.ctr_lon == None):
+        log.error("sector 'meso' requires --ctr-lat & --ctr-lon")
+        exit(1)
+
     try:
         start_time, end_time = get_start_end(args.filenames)
     except: