fail gracefully if unihan.db is not available

anki/features/chinese/__init__.py

@@ -57,11 +57,14 @@ class ChineseGenerator(object):
         self.unihan = None
 
     def toReading(self, type, val):
+        try:
             if not self.unihan:
                 self.unihan = UnihanController(type)
             else:
                 self.unihan.type = type
             return self.unihan.reading(val)
+        except:
+            return u""
 
 unihan = ChineseGenerator()