Show warning when too many stars couldn't have their light curve generated

[vterron]

The command diffphot should warn the users when there are too many astronomical objects whose light curve could not be generated because of an insufficient number of images or comparison stars. It is confusing for users to see that the command has apparently succeeded, only to find out later, with juicer, that most (if not all) of the objects have no light curve — usually because of the default value of options --minimum-images and --minimum-stars. At the very least we should warn the user that there's probably something wrong with these values.