the CLUSTER_USERNAME variable was not being used when running from the
frontend.

since the /tmp directory is shared by everyone then some left over files
remain. when a different user attempts to remove or write to those files
but does not have permission it causes failures. this commit just uses
/tmp/$USER instead of using /tmp directly

when a job is not yet in running state the program runs but the function
that lists the job's machines returns zero machines which can cause a
panic from division by 0 which is not a very helpful message. this
commit improves the error/warn messages when no machines are listed for
a job.