Add custom exception for connection errors

[kaisero]

Currently if Connection is refused by FMC (e.g. not reachable) ConnectionError is thrown.

ConnectionError: HTTPSConnectionPool(host='fmc.example.com', port=443): Max 
retries exceeded with url: /api/fmc_platform/v1/auth/generatetoken (Caused by 
NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x10dc3f490>: 
Failed to establish a new connection: [Errno 61] Connection refused'))

This should be replaced with a more user friendly custom exception