Loft and shell failure should show why

Loft and shell operations routinely fail due to the precise dimensions/positions of the relevant bodies.

Currently, the user must pursue a trial-and-error approach (eg. via progressively slicing bits off a body) to discover which parts of a body are preventing the operation.

When a loft or shell operation fails, the regions of the relevant bodies preventing the operation should be marked (eg coloured red), to indicate how to resolve the issue.