RefreshWithMFAUnsupportedError: Cannot refresh credentials: MFA token required.

Awslogs may crash if MFA credentials expire while it is getting a large log stream.

Trace:

```
Version: 0.12.0
Python: 3.7.3 (default, Mar 27 2019, 09:23:32)
[Clang 9.0.0 (clang-900.0.39.2)]
boto3 version: 1.10.45
Platform: Darwin-16.7.0-x86_64-i386-64bit
Args: ['/usr/local/bin/awslogs', 'get', '', '', '-s', '52 weeks ago']
Config: {'aws_access_key_id': 'SENSITIVE',
'aws_profile': 'SENSITIVE',
'aws_region': None,
'aws_secret_access_key': 'SENSITIVE',
'aws_session_token': 'SENSITIVE',
'color': 'auto',
'end': None,
'filter_pattern': None,
'func': 'list_logs',
'log_group_name': '',
'log_stream_name': '',
'output_group_enabled': True,
'output_ingestion_time_enabled': False,
'output_stream_enabled': True,
'output_timestamp_enabled': False,
'query': None,
'start': '52 weeks ago',
'watch': False,
'watch_interval': 1}

Traceback (most recent call last):
File "/usr/local/lib/python3.7/site-packages/awslogs-0.12.0-py3.7.egg/awslogs/bin.py", line 178, in main
getattr(logs, options.func)()
File "/usr/local/lib/python3.7/site-packages/awslogs-0.12.0-py3.7.egg/awslogs/core.py", line 223, in list_logs
consumer()
File "/usr/local/lib/python3.7/site-packages/awslogs-0.12.0-py3.7.egg/awslogs/core.py", line 164, in consumer
for event in generator():
File "/usr/local/lib/python3.7/site-packages/awslogs-0.12.0-py3.7.egg/awslogs/core.py", line 151, in generator
response = self.client.filter_log_events(**kwargs)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/client.py", line 276, in _api_call
return self._make_api_call(operation_name, kwargs)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/client.py", line 573, in _make_api_call
operation_model, request_dict, request_context)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/client.py", line 592, in _make_request
return self._endpoint.make_request(operation_model, request_dict)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/endpoint.py", line 102, in make_request
return self._send_request(request_dict, operation_model)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/endpoint.py", line 132, in _send_request
request = self.create_request(request_dict, operation_model)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/endpoint.py", line 116, in create_request
operation_name=operation_model.name)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/hooks.py", line 356, in emit
return self._emitter.emit(aliased_event_name, **kwargs)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/hooks.py", line 228, in emit
return self._emit(event_name, kwargs)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/hooks.py", line 211, in _emit
response = handler(**kwargs)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/signers.py", line 90, in handler
return self.sign(operation_name, request)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/signers.py", line 152, in sign
auth = self.get_auth_instance(**kwargs)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/signers.py", line 232, in get_auth_instance
frozen_credentials = self._credentials.get_frozen_credentials()
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/credentials.py", line 591, in get_frozen_credentials
self._refresh()
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/credentials.py", line 486, in _refresh
self._protected_refresh(is_mandatory=is_mandatory_refresh)
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/credentials.py", line 502, in _protected_refresh
metadata = self._refresh_using()
File "/usr/local/lib/python3.7/site-packages/botocore-1.13.45-py3.7.egg/botocore/credentials.py", line 248, in __call__
raise RefreshWithMFAUnsupportedError()
botocore.exceptions.RefreshWithMFAUnsupportedError: Cannot refresh credentials: MFA token required.
```

Start with awslogs/core.py's consumer and generator around the filter_log_events call shown in the trace, using the reported long-stream scenario to investigate credential expiry. Check awslogs/bin.py's main path for how the exception reaches the CLI. Done means the MFA-expiry case has defined, tested behavior without an unhandled crash.

Written by the indexing model from the issue text.