"bad key in cancel request"

I noticed that processCancelRequest() emits a log message at DEBUG2 when
it receives a cancel request with a bad key or for a non-existent PID.
For example,

    ereport(DEBUG2,
            (errmsg_internal("bad key in cancel request for process %d",
             backendPID)));

I think this ought to be logged at a higher level than DEBUG2: for one
thing, it is a potential security issue the DBA might want to be aware
of. It could also indicate a misconfigured or buggy client application.
Therefore, I think we should raise the level to WARNING, or perhaps
NOTICE. Any objections?

-Neil