Move handling of fatal errors in slurm.conf parse into single location

This will make error handling simpler as we want API users to not
see a fatal error/exit with a bad configuration, but only an error.
bug 374 (movement toward a fix)
Avoid fatal error/exit from API functions

Generate an error return rather than fatal error and exit if the
configuraiton file is absent or invalid. This will permit Slurm APIs to be
more reliably used by other programs.
bug 374