Fix bug in reporting missing keys in contact imports

```
Stacktrace (most recent call last):

  File "celery/task/trace.py", line 233, in trace_task
    R = retval = fun(*args, **kwargs)
  File "celery/task/trace.py", line 420, in __protected_call__
    return self.run(*args, **kwargs)
  File "go/contacts/tasks.py", line 382, in import_upload_is_truth_contacts_file
    fields, has_header)
  File "go/contacts/tasks.py", line 338, in import_and_update_contacts
    errors.append((key, 'No key provided'))
```