Raise exception if not connected

[erikbosch]

Fixes #667

With this change, if calling authorize before connect you get:

erik@debian3:~/kuksa.val/kuksa-client$ ./hej.py 
Traceback (most recent call last):
  File "/home/erik/kuksa.val/kuksa-client/./hej.py", line 105, in <module>
    client.authorize(token)
  File "/home/erik/kuksa.val/kuksa-client/kuksa_client/grpc/__init__.py", line 678, in wrapper
    raise Exception("Server not connected! Call connect() before using this command!")
Exception: Server not connected! Call connect() before using this command!
erik@debian3:~/kuksa.val/kuksa-client$ 

PR consist of two types of changes:

• Exception instead of info for both sync and async
• Alignment of connection check, previously connection was checked just for subset

[erikbosch]

PR extended to also cover async. Also aligned connection check for all methods

[erikbosch]

Need to do some more testing