Pass message instead of object representation

Exceptions implement `.__str__` which can be used to be passed to the template error output. This prevents ugly string formatting like `u''`.

import_export/resources.py

@@ -341,7 +341,7 @@ def import_data(self, dataset, dry_run=False, raise_errors=False,
         except Exception as e:
             logging.exception(e)
             tb_info = traceback.format_exc()
-            result.base_errors.append(Error(repr(e), tb_info))
+            result.base_errors.append(Error(e, tb_info))
             if raise_errors:
                 if use_transactions:
                     savepoint_rollback(sp1)