a-delete freezes on empty bucket without --rmb

Had an empty bucket `ace_runs` which I tried to delete but a-delete seemed to freeze:
```
$ a-delete ace_runs
swift connection OK
Object name: ace_runs not found in allas.
^C
$ Traceback (most recent call last):
File "/appl/bin/swift", line 6, in
from swiftclient.shell import main
File "/appl/soft/bio/bioconda/miniconda3/lib/python3.7/site-packages/swiftclient/__init__.py", line 20, in
from .client import * # noqa
File "/appl/soft/bio/bioconda/miniconda3/lib/python3.7/site-packages/swiftclient/client.py", line 65, in
from keystoneclient import exceptions as ksexceptions
File "", line 1032, in _handle_fromlist
File "/appl/soft/bio/bioconda/miniconda3/lib/python3.7/site-packages/keystoneclient/__init__.py", line 72, in __getattr__
return importlib.import_module('keystoneclient.%s' % name)
File "/appl/opt/anaconda3/2019.3/envs/default/lib/python3.7/importlib/__init__.py", line 127, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "", line 1006, in _gcd_import
File "", line 983, in _find_and_load
File "", line 967, in _find_and_load_unlocked
File "", line 677, in _load_unlocked
File "", line 724, in exec_module
File "", line 818, in get_code
File "", line 917, in get_data
KeyboardInterrupt
^C
```

`$ a-delete --rmb ace_runs` worked though.