Improve argument checking for `stage_in`

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\
)
This typically refers to a problem with your application.
Please see the FAQ page for debugging suggestions

```