Prevent a crash when updating pootle-terminology.po

When the file does not exist on disk, accessing attributes on
self.file will fail. So check for that in advance. If this happens
on anything else than pootle-terminology.po, log a warning.

Fixes #3254

pootle/apps/pootle_store/models.py

@@ -1597,6 +1597,13 @@ def update(self, update_structure=False, update_translation=False,
             self.parse(store=store)
             return
 
+        if not self.file:
+            # This will always happen for pootle-terminology.po, don't warn
+            if not self.pootle_path.endswith("pootle-terminology.po"):
+                logging.warning(u"Attempted to update a non-existing file",
+                                self.pootle_path)
+            return
+
         if only_newer:
             disk_mtime = datetime.datetime \
                                  .fromtimestamp(self.file.getpomtime()[0])