Load previous valid SLURM on any error, validate tal/slurm conf args.

Load previous valid SLURM on any error, validate tal/slurm conf args.

+Previous valid SLURM was applied only if a newer SLURM had syntax errors; this has changed, now it's applied on any error.
+Log error when the version isn't set at SLURM file.
+Validate configured location (can be a file or directory) of 'tal' and 'slurm' args when the application starts.