SMB ignore connection errors when NetBIOS is disabled

When NetBIOS is blocked or disabled, a smb::look() call can still get
the list of shares from smbclient, even though the name resolution at
the end of the command fails.

This fix makes it ignore the connection error if we already got the list
of shares.

Fixed #5860
Merge pull request #5916 from owncloud/extstorage-smb-ignorenetbioserror

SMB ignore connection errors when NetBIOS is disabled