pbs: update connection failed error message

* Addresses #6531 on the 8.4.x branch.
* Cylc 8 port of 7b6d8cb
* Update the PBS job runner to reflect a change in error message.
* The new pattern will also match the old format.

Author: oliver-sanders

changes.d/6586.fix.md

@@ -0,0 +1,2 @@
+Update PBS job runner to reflect error message change. This change
+continues to support older PBS versions.

cylc/flow/job_runner_handlers/pbs.py

@@ -81,7 +81,7 @@ class PBSHandler:
     # N.B. The "qstat JOB_ID" command returns 1 if JOB_ID is no longer in the
     # system, so there is no need to filter its output.
     POLL_CMD = "qstat"
-    POLL_CANT_CONNECT_ERR = "Connection refused"
+    POLL_CANT_CONNECT_ERR = "cannot connect to server"
     REC_ID_FROM_SUBMIT_OUT = re.compile(r"^\s*(?P<id>\d+)", re.M)
     SUBMIT_CMD_TMPL = "qsub '%(job)s'"
     TIME_LIMIT_DIRECTIVE = "-l walltime"