Resolve "make error messages more descriptive"

Closes #8

gridded_glm/libexec/_minute_gridder.py

@@ -152,6 +152,11 @@ 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)
+
     try:
         start_time, end_time = get_start_end(args.filenames)
     except ValueError: