Produce empty array when no ordered NTS exist.

We should produce an empty array when we can't find the ordered nucleotides,
otherwise this causes an error.
Add basic warning when no 2D can be drawn.

The warning will only be displayed if there are no ordered nucleotides. In this
case we can't even draw a backbone so we just put up a warning instead.
Fail nicely when no interactions are known.

In this case we can still draw a backbone so we do that, even though there are
no interactions.
Trigger rendering.

Last commit disabled actually drawing the diagram. This fixes that.
More informative message when no nts found.
Inform when no interactions found.

If no interactions are found then we disable all buttons and display a warning.
Use a bootstrap alert for error messages.

When we display a warning that no interactions are found we use a bootstrap
error alert to do so.
Correct interactions url.
If given an invalid PDB in 2D display an error.

This creates an invalid pdb view and uses it when given a PDB which is invalid.
It may be a good idea to use this view for all invalid PDB views.
Don't close container to display footer correctly