warn user when no columns available to compute structure superposition

When a 'view all structures' or 'Align in Jmol' operation is applied to a set of structures which have no aligned residues in any associated views, then the structure superposition process fails but no indication of what went wrong is reported to the user.

A dialog box (or status bar notification) should be shown to tell the user that the superposition couldn't be performed due to the lack of aligned residues.