Fix #95: catch OSError when collecting file descriptors. (#205)

Author: blopker

prometheus_client/process_collector.py

@@ -83,7 +83,7 @@ def collect(self):
                                               'Number of open file descriptors.',
                                               len(os.listdir(os.path.join(pid, 'fd'))))
             result.extend([open_fds, max_fds])
-        except IOError:
+        except (IOError, OSError):
             pass
 
         return result