Improve warning message in pg_signal_backend()

Improve warning message in pg_signal_backend()

Previously, invoking pg_terminate_backend() or pg_cancel_backend()
with the postmaster PID produced a "PID XXXX is not a PostgresSQL
server process" warning, which does not make sense. Change to
"backend process" to make the message more exact.

Nathan Bossart, based on an idea from Bharath Rupireddy with
input from Tom Lane and Euler Taveira

Discussion: https://www.postgresql.org/message-id/flat/CALj2ACW7Rr-R7mBcBQiXWPp=JV5chajjTdudLiF5YcpW-BmHhg@mail.gmail.com