merge_config.sh: exit on missing input files

Add a check for the existence of input files and exit (with failure)
if they are missing.

Without this additional check, missing files produce error messages
but still result in an output file being generated and a successful
exit code.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Michal Marek <mmarek@suse.com>