If a pid disappears on us while we're reading, we should just continue
on.
EnvironmentError is just an alias for OSError since Python 3.3, so use
the latter name. [0]
[0] https://docs.python.org/3/library/exceptions.html#OSError
Change-Id: I3a25cca328e1469f72c84a118a9691c1c0258bc4
Closes-Bug: #1926434