httppeer: make __del__ access to self.urlopener more safe
Some errors could in some cases show unfortunate scary and confusing warnings
from the httppeer delstructors:
abort: nodename nor servname provided, or not known
Exception AttributeError: "'httpspeer' object has no attribute 'urlopener'" in <bound method httpspeer.__del__ of <mercurial.httppeer.httpspeer object at 0x
106e1f5d0>> ignored```
To mute that, take
7b15dd9125b3 to the next level and use getattr in __del__.
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