alignak-backend-cli displays a traceback when can't talk to backend

[mohierf]

From @arthurlogilab, initially opened as Alignak-monitoring-contrib/alignak-webui#275

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out of the box I don't think alignak-backend-cli should display a python traceback to the user when it can't reach the default backend.

$ alignak-backend-cli 
INFO:alignak_backend_client.client:Authenticating...
ERROR:alignak_backend_client.client:Backend connection error, error: HTTPConnectionPool(host='127.0.0.1', port=5000): Max retries exceeded with url: /login (Caused by NewConnectionError('<requests.packages.urllib3.connection.HTTPConnection object at 0x7ff04afb8690>: Failed to establish a new connection: [Errno 111] Connection refused',))
ERROR:alignak_backend_client.client:Exception: Backend error code 1000: Backend connection error
Traceback (most recent call last):
  File "/home/arthur/.virtualenvs/alignak/local/lib/python2.7/site-packages/alignak_backend_client/backend_client.py", line 398, in initialize
    self.backend.login(self.username, self.password)
  File "/home/arthur/.virtualenvs/alignak/local/lib/python2.7/site-packages/alignak_backend_client/client.py", line 176, in login
    raise BackendException(1000, "Backend connection error")
BackendException: Backend error code 1000: Backend connection error
ERROR:alignak_backend_client.client:Response: None
Access denied!
~~~~~~~~~~~~~~~~~~~~~~~~~~
Exiting with error code: 1