Improve the error message when required flags aren't set for inference.py.
Improve the error message when required flags aren't set for
inference.py.

Change-Id: I3a6a1d99d64cc33fb5e4fe4b5c5238a9b5e0ac15

hxkk/u8m_test,coded5282/youtube-8m,lidalei/youtube-8m,slai11/youtube-8m,KangHsi/youtube-8m,forwchen/yt8m,slai11/youtube-8m,slai11/youtube-8m,google/youtube-8m,mwoodson1/youtube-8m-competition,slai11/youtube-8m,KangHsi/youtube-8m,forwchen/yt8m