git annex add silently ignores non-existent files

[Adam Spiers]

    git annex add this_file_does_not_exist

does not result in a warning.  This leads to confusing (lack of)
behaviour in certain cases, e.g.

    generate_a_list_of_files_some_of_which_contain_spaces | xargs git annex add

would silently fail to add the files containing spaces (because you'd
need the -d or -0 switch to xargs to cope with this case).