[easy] Fix zstd bench error message

[senhuang42]

I remember this confusing me the first time I saw it, so should probably fix it.
Prior to this fix, if you ran

./zstd -b1 nonexistent_file
Not enough memory; testing 0 MB only...
Error 12 : not enough memory 

Now the message is more accurate

Error 9 : Error loading files

[Cyan4973]

Thanks @senhuang42 ! This is indeed a small yet welcome improvement !