Improve argument checking for `stage_in`

[hyoklee]

Garbage input generates Segmentation fault.

[hyoklee@jelly]/scr/hyoklee/src/hermes/build/data_stager/test/Testing/Temporary\
> mpirun -n 1 stage_in 2 3 4 5
WARNING: Logging before InitGoogleLogging() is written to STDERR
I1212 08:08:57.493853 27855 hermes.cc:419] Initializing hermes config
shm_open failed: No such file or directory

===============================================================================\
====
=   BAD TERMINATION OF ONE OF YOUR APPLICATION PROCESSES
=   PID 27855 RUNNING AT jelly
=   EXIT CODE: 139
=   CLEANING UP REMAINING PROCESSES
=   YOU CAN IGNORE THE BELOW CLEANUP MESSAGES
===============================================================================\
====
YOUR APPLICATION TERMINATED WITH THE EXIT STRING: Segmentation fault (signal 11\
)
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