bundle2: comment to clarify why the handler call is where it is

The reason why it is here is not obvious. I'm the one who wrote it there in the
first place and almost moved it 2 weeks later.

mercurial/bundle2.py

@@ -305,6 +305,11 b' def processbundle(repo, unbundler, trans'
                 # todo:
                 # - consume the part once we use streaming
                 continue
+
+            # handler is called outside the above try block so that we don't
+            # risk catching KeyErrors from anything other than the
+            # parthandlermapping lookup (any KeyError raised by handler()
+            # itself represents a defect of a different variety).
             handler(op, part)
     except Exception:
         for part in iterparts: