Mojo: Make the shell's loader log URL fetch errors.

This is to help debugging and make various failures less mysterious.

R=darin@chromium.org, abarth@chromium.org

Review URL: https://codereview.chromium.org/132283003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243976 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243976