tracereport: also catch gaierror for gethostbyaddr()

socket.gethostbyaddr() throws a socket.gaierror exception
if the address can not be resolved which was not caught before.

Fixes regression introduced by e7d7c6f , after which
Pilot yielded stage-in and stage-out errors with backtrace
for sites using a non-DNS resolvable PANDA_HOSTNAME
(VMs, long-lived containers,...).

pilot/util/tracereport.py

@@ -115,7 +115,7 @@ def init(self, job):
 
         try:
             self['hostname'] = socket.gethostbyaddr(hostname)[0]
-        except socket.herror as exc:
+        except (socket.gaierror, socket.herror) as exc:
             logger.warning(f'unable to detect hostname by address for trace report: {exc}')
             self['hostname'] = 'unknown'