Do not crash if SExtractor did not return the requested checkimage.

It is anyway bad as it means the code is run incorrectly or SExtractor failed, but at least we should not add our own crash on top of that!

stdpipe/photometry.py

@@ -383,7 +383,11 @@ def get_objects_sextractor(image, header=None, mask=None, err=None, thresh=2.0,
         result = [result]
 
         for name in checknames:
-            result.append(fits.getdata(name))
+            if os.path.exists(name):
+                result.append(fits.getdata(name))
+            else:
+                log("Cannot find requested output checkimage file", name)
+                result.append(None)
 
     if _workdir is None:
         shutil.rmtree(workdir)