Check if configuration is available

[mbucas]

When no configuration file is given, you get a traceback instead of a clean error message :

Traceback (most recent call last):
  File "/opt/bin/Route53Cmd.py", line 228, in <module>
    main()
  File "/opt/bin/Route53Cmd.py", line 225, in main
    command.run()
  File "/opt/bin/Route53Cmd.py", line 98, in run
    self.do_list()
  File "/opt/bin/Route53Cmd.py", line 129, in do_list
    rs = self.getRecordSets()
  File "/opt/bin/Route53Cmd.py", line 122, in getRecordSets
    for zone in self.r53.list_hosted_zones():
  File "/usr/lib64/python2.7/site-packages/route53/connection.py", line 87, in _do_autopaginating_api_call
    root = self._send_request(path, params, method)
  File "/usr/lib64/python2.7/site-packages/route53/connection.py", line 47, in _send_request
    response_body = self._transport.send_request(path, data, method)
  File "/usr/lib64/python2.7/site-packages/route53/transport.py", line 105, in send_request
    headers = self.get_request_headers()
  File "/usr/lib64/python2.7/site-packages/route53/transport.py", line 76, in get_request_headers
    signing_key = self._hmac_sign_string(date_header)
  File "/usr/lib64/python2.7/site-packages/route53/transport.py", line 58, in _hmac_sign_string
    self.connection._aws_secret_access_key.encode('utf-8'),
AttributeError: 'NoneType' object has no attribute 'encode'