Trac #21800: Better error handling in sage-cleaner

My `sage-cleaner` instance is randomly killing jobs. Reason:
{{{
Checking PIDs [18654]
Process 18654 is no longer running, so we clean up
Killing 18654's spawned jobs
--> Killing 'gp' with PID 18743 and parent PID 18654
--> Killing 'gp' with PID 18759 and parent PID 18654
--> Killing 'gp' with PID 18841 and parent PID 18654
--> Killing 'gp' with PID 18851 and parent PID 18654
--> Killing 'gp' with PID 18868 and parent PID 18654
--> Killing 'gp' with PID 18878 and parent PID 18654
--> Killing 'gp' with PID 18982 and parent PID 18654
--> Killing 'gp' with PID 19333 and parent PID 18654
Exception while cleaning up PID 18654:
Traceback (most recent call last):
  File "/usr/local/src/sage-config/src/bin/sage-cleaner", line 94, in
cleanup
    or kill_spawned_jobs(spawned_processes, parent_pid):
  File "/usr/local/src/sage-config/src/bin/sage-cleaner", line 106, in
kill_spawned_jobs
    pid, cmd = job.strip().split(' ', 1)
ValueError: need more than 1 value to unpack
}}}

Probably the `jobfile` got corrupted somehow and we need to handle this
gracefully.

URL: https://trac.sagemath.org/21800
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Frédéric Chapoton

src/bin/sage-cleaner

@@ -103,9 +103,13 @@ def kill_spawned_jobs(jobfile, parent_pid):
     logger.info("Killing %s's spawned jobs", parent_pid)
     killed_them_all = True
     for job in open(jobfile).readlines():
-        pid, cmd = job.strip().split(' ', 1)
-        logger.info("--> Killing '%s' with PID %s and parent PID %s", cmd, pid, parent_pid)
-        pid = int(pid)
+        try:
+            pid, cmd = job.strip().split(' ', 1)
+            pid = int(pid)
+            logger.info("--> Killing %r with PID %s and parent PID %s", cmd, pid, parent_pid)
+        except Exception:
+            logger.error("Exception while processing job %r from %s, ignoring", job, jobfile)
+            continue
         try:
             pgrp = os.getpgid(pid)
             logger.info("--> Killing process group %s", pgrp)