Catch exception while clearing TCP conn to broker

When a DISCONNECT is sent to the broker, we proactively close the TCP
connection unless the DISCONNECT has a receipt header. Depending on
the timing, the broker may also close the connection on its side.
That appears to cause an exception in reactor on the CI server, e.g.:

https://build.spring.io/browse/SPR-PUB-JOB1-1715/test/case/135247530

This change traps the exceptions and prevents it from propagating.