Properly handle exceptions with subprocess calls in the Slurm scheduler.

[Paul-Ferrell]

• subprocess.check_output will raise a CalledProcessError if it returns non-zero. These should be handled.
• stderr isn't redirected properly on such calls.
• decoding the output can raise an error too. These should be handled.
• Double check that all other subprocess calls are properly wrapped (most won't need the exception handling, but all need to redirect output properly.)