Resolve "make error messages more descriptive"

Closes #8

gridded_glm/libexec/_minute_gridder.py

@@ -162,6 +162,9 @@ def grid_setup(args, work_dir=os.getcwd()):
     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.")
+        exit(1)
 
     base_date = datetime(start_time.year, start_time.month, start_time.day)
     proj_name = 'geos'