smb3 client: warn when parse contexts returns error on compounded operation

Coverity noticed that the rc on smb2_parse_contexts() was not being checked
in the case of compounded operations.  Since we don't want to stop parsing
the following compounded responses which are likely valid, we can't easily
error out here, but at least print a warning message if server has a bug
causing us to skip parsing the open response contexts.

Addresses-Coverity: 1639191
Acked-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>