isse #776: print an error when no given font is a TTF
isse #776: print an error when no given font is a TTF

moyogo/fontbakery,googlefonts/fontbakery,graphicore/fontbakery,moyogo/fontbakery,googlefonts/fontbakery,moyogo/fontbakery,graphicore/fontbakery,graphicore/fontbakery,googlefonts/fontbakery