exchange: propagate the subfunctions return

The parts generator can return a callback to handle server reply. We should
propagate the return for correctness.

mercurial/exchange.py

@@ -813,9 +813,9 b' def _pushb2phases(pushop, bundler):'
     hasphaseheads = 'heads' in b2caps.get('phases', ())
 
     if hasphaseheads and not legacyphase:
-        _pushb2phaseheads(pushop, bundler)
+        return _pushb2phaseheads(pushop, bundler)
     elif haspushkey:
-        _pushb2phasespushkey(pushop, bundler)
+        return _pushb2phasespushkey(pushop, bundler)
 
 def _pushb2phaseheads(pushop, bundler):
     """push phase information through a bundle2 - binary part"""