Be more careful when checking for udisks-iscsi availability
Be more careful when checking for udisks-iscsi availability

If udisks (storaged) is not installed at all, we may get an error
even when trying to find out if udisks (storaged) provides the
iSCSI functionality via its Manager object.

Resolves: rhbz#1429132

rvykydal/blivet,rvykydal/blivet,rhinstaller/blivet,rhinstaller/blivet,vojtechtrefny/blivet,vojtechtrefny/blivet,jkonecny12/blivet,jkonecny12/blivet