Report an error if no fits are provided for a restraint

[benmwebb]

If the user did not provide any model-to-restraint fits for a given restraint, this is a probably a user error and the dumper should report about it (since this means the restraint wasn't used anywhere). With some restraints that use a single table (such as ihm_sas_restraint) having no fits means that the restraint won't be recorded in the mmCIF file at all. (Other restraints are split into a restraint info table like ihm_2dem_class_average_restraint and a fit table like ihm_2dem_class_average_fitting, so at least the info table will still be written even if no fits are available.)