add clearer exception

s2m/abstract_study.py

@@ -311,6 +311,7 @@ class AbstractStudy(object):
     def ordered_years_and_path_files(self):
         nc_files = [(int(f.split('_')[-2][:4]) + 1, f) for f in os.listdir(self.study_full_path) if f.endswith('.nc')]
         assert op.exists(self.study_full_path)
+        assert len(nc_files) > 0
         ordered_years, path_files = zip(*[(year, op.join(self.study_full_path, nc_file))
                                           for year, nc_file in sorted(nc_files, key=lambda t: t[0])
                                           if (self.year_min <= year <= self.year_max)