skytools.DBScript: safer pidfile writing
skytools.DBScript: safer pidfile writing

- signal_pidfile: Clarify ValueError error message. Thrown usually
  on empty pidfiles, the error messega will now mention the pidfile name.

- run_single_process: restructure pidfile writing, so that
  the pidfile is removed if .write() failed, but not when open() failed.
  This should avoid the chance that empty pidfiles are hanging around.

overdrive3000/skytools,markokr/skytools,tarvip/skytools,tarvip/skytools,overdrive3000/skytools,tarvip/skytools,overdrive3000/skytools,markokr/skytools,markokr/skytools