do not continue when an invalid service is loaded

openlp/core/lib/serviceitem.py

@@ -636,14 +636,17 @@ class ServiceItem(object):
         for frame in self._raw_frames:
             if self.is_image() and not os.path.exists(frame[u'path']):
                 self.is_valid = False
+                break
             elif self.is_command():
                 file_name = os.path.join(frame[u'path'], frame[u'title'])
                 if not os.path.exists(file_name):
                     self.is_valid = False
+                    break
                 if suffix_list and not self.is_text():
                     file_suffix = frame[u'title'].split(u'.')[-1]
                     if file_suffix.lower() not in suffix_list:
                         self.is_valid = False
+                        break
 
     def _get_renderer(self):
         """