Ignore EPIPE in pipefilter
This hides the following traceback (there's a race condition involved,
so you may have to try a few times to hit it):
$ hg sign --key key-that-does-not-exist
Signing 2062:
4bad92f4ea65
gpg: skipped "key-that-does-not-exist": secret key not available
gpg: signing failed: secret key not available
Exception in thread Thread-1:
Traceback (most recent call last):
File "threading.py", line 442, in __bootstrap
self.run()
File "threading.py", line 422, in run
self.__target(*self.__args, **self.__kwargs)
File "mercurial/util.py", line 24, in writer
pin.close()
IOError: [Errno 32] Broken pipe
abort: Error while signing