Add except path with exception debug to send_recv

The related bug resulted when an exception occurred within the
future.result() call. This caused the finally block to be executed,
and therefore myid to be deleted from self.outstanding_msgs prior
to _reader_main() checking if the msgid not in self.outstanding_msgs.
This caused _reader_main() to raise an AssertionError because the
msgid was no longer in outstanding_msgs. This is a small step forward
to log a warning when this siutation occurs.

Related-Bug: #1927868
Change-Id: I2eed242e0c796b8a2aa3d1b21bd1da4c497f624d

oslo_privsep/comm.py

@@ -24,6 +24,7 @@ import datetime
 import enum
 import logging
 import socket
+import sys
 import threading
 
 import msgpack
@@ -186,6 +187,9 @@ class ClientChannel(object):
                 self.writer.send((myid, msg))
 
                 reply = future.result()
+            except Exception:
+                LOG.warning("Unexpected error: {}".format(sys.exc_info()[0]))
+                raise
             finally:
                 del self.outstanding_msgs[myid]