main : check if input files exist before proceeding (whisper/1872)

main : check if input files exist before proceeding (whisper/1872)

Until the most recent commit (3d42463), the main.cpp sample file does
not check whether the input files exist or not. Consequently, the
model is loaded first before reporting whether there was a failure or
not when processing a file. In environments with HDD, this can take
about 50 seconds or more, depending on the loaded model.

This commit addresses this issue by checking in advance whether the
input files exist or not.