Update some preset-specific error messages

[ljubica-milovic]

Background

This ticket is for updating the error messages from a previous ticket.

Requirement

1. Error message "Every atom in the loaded structure must belong to a sugar, base, or a phosphate." should be "Some atoms do not belong to any nucleotide component (sugar, base, or phosphate)."

2. Error message "Every atom in the loaded structure must belong only to one of the components (sugar, base, or phosphate)." should be "Some atoms belong to multiple nucleotide components."

3. Error message "Every preset must contain at least two components, with sugar being a mandatory component." should be "Preset must contain at least two components, with sugar being one of them."