To repair the holes purchase a small container of non shrinking spackle paste, surgical or particle mask, safety glasses, 150 grade sandpaper, Masonry drill bit that is just smaller than your masonry wall anchor, 4 inch plaster knife, plaster weld, a disposable chipper brush 1".
Remove any loose plaster by scraping with a sturdy putty knife or painters 5 way tool also known as a five in one ( be sure to wear your dust mask and safety glasses.
Once you've removed all loose plaster you should vacuum holes and apply plaster weld to any areas where the plaster or structolite is visible, when the plaster weld liquid has set up you can then apply the spackle paste and be sure to push the spackle paste in as far as possible into the screw holes.
In many cases two coats of spackling paste is needed, let the 1st coat dry completely (2 or 3 hours) before applying second coat. once the spackle has dried and cured you may sand spackle smooth and be sure to vacuum the dust with a good hepa filter shop vac. when rehanging your curtains you should mark your holes with a pencil making sure your holes are level on the opposite side- you can drill your new pilot holes and insert your masonry wall anchors after.
One last but important note: if your home was built before 1980 it would be a good idea to call in a professional or take a lead tester to the area being attempting any work.