Raise exception when ContinueRetrievePropertiesEx used before RetrievePropertiesEx

[hartsock]

In a given session, you should be able to track when a call to RetrievePropertiesEx is made and leave a state flag on the session instance. When the call to ContinueRetrievePropertiesEx is made you should be able to observe the flag and throw a Session related exception.

This problem occurs because we have automatic re-connects embedded inside pyVmomi's core libraries and those reconnects can occasionally cause problems with a long running retrieve.