Resolve "make error messages more descriptive"

Closes #8

gridded_glm/libexec/_minute_gridder.py

@@ -152,11 +152,21 @@ def grid_setup(args, work_dir=os.getcwd()):
             exit(0)
         args.filenames = fileglob
 
+    for f in args.filenames:
+        if not os.path.exists(f):
+            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 ValueError:
-        log.error("Non-standard filenames provided, use --start and --end to specify data times.")
-        raise
+    except:
+        log.error("Could not parse start & end times from one or more of the files provided:")
+        log.error(", ".join(args.filenames))
+        exit(1)
 
     base_date = datetime(start_time.year, start_time.month, start_time.day)
     proj_name = 'geos'