Add slurm_internal error code
Make job modify message more generic

This is important for job array modifications. Log the job ID in the job record rather than in the modify request.
check validity of job array record to copy
job cancel request fix

Previously would not work properly if a job array to be canceled was partly running and partly pending
Add job array modification logic

This is the infrastructure needed to modify a job array with the
new job array data structures. There will need to be additional
work once the RPC is fleshed out.