Print finder when no files are found

makes it instantaneous to debug the finder / fixes

lib/data/__init__.py

@@ -56,6 +56,8 @@ def create_data(module, pregex, get_root, ARGS_DIR,
             open_mf_kw_def['preprocess'] = finder.get_func_process_filename(
                 open_mf_kw_def.pop('preprocess_finder'), *pp_args, **pp_kw)
 
+        if len(finder.files) == 0:
+            raise RuntimeError("No files found. \n{:s}".format(str(finder)))
         ds = xr.open_mfdataset(finder.get_files(), **open_mf_kw_def)
         return ds