saltutil: handle IOError in running()

The running() function was unreliable, because it would
fail on IOErrors reading proc files.  Those are actually
really common because the proc files are being created
and destroyed in the background while jobs run.

This made kill_job unreliable too, because it relies on
running() to find its victim.
Merge pull request #13109 from jcsp/saltutil-running-enoent

saltutil: handle IOError in running()