Clearer error message if pyelftools is not installed
Clearer error message if pyelftools is not installed

R=ppi@chromium.org, ppi@google.com

Review URL: https://codereview.chromium.org/1251493003 .

afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo,afandria/mojo