Implemented fix for failed cursor iteration (olivierkes#729)

A long time ago, I identified this failure but wasn't sure why it was
failing. I still don't know why, but the emergency fix at least prevents
other people from running into it. As a bonus, since we have a proper
logging facility now, we can actually log it as I desired to do back
then!

This commit tentatively fixes issue 729.

manuskript/ui/views/MDEditView.py

@@ -500,7 +500,10 @@ def getClickRects(self):
                         r3.setLeft(self.viewport().geometry().left())
                         r3.setRight(self.viewport().geometry().right())
                         refs.append(ClickThing(r3, rx, rx.capturedTexts()))
-                        cursor.movePosition(cursor.Down)
+                        if not cursor.movePosition(cursor.Down):
+                            # Super-rare failure. Leaving log message for future investigation.
+                            LOGGER.debug("Failed to move cursor down while calculating clickables. Aborting.")
+                            break
 
         self.clickRects = refs