Suggested changes xtrabackup plugin

Agree, however, I'm not sure that the 9 exit code is the right one here.
I've never been there or seen such a scenario, but potentially it may
happen.

Co-authored-by: sduehr <stephan.duehr@bareos.com>

core/src/plugins/filed/python/percona-xtrabackup/bareos-fd-percona-xtrabackup.py

@@ -597,6 +597,8 @@ def end_dumper(self):
                         "Giving up to wait for bareos_xtrabackup_dumper PID %s to terminate\n"
                         % (self.stream.pid),
                     )
+                    # Set return code to 9 because we've tried to terminate subprocess earlier
+                    self.stream.returncode = 9
                     break
             DebugMessage(
                 100,