Bug 556357: add c_modeless to addmodeline script (r=fklockii).
Bug 556357: add c_modeless to addmodeline script (r=fklockii).

My end goal is to add mode lines to the .as code in test/performance.
We don't have a standard mode setting for Actionscript code (some
files use C++, some use Java.)

Rather than introduce a convention in a drive-by patch, I'll just
leave the Mode out.

(Also put in some drive-by improvements to the script itself,
e.g. messages explaining error exits).

pfgenyun/tamarin-redux,pfgenyun/tamarin-redux,pnkfelix/tamarin-redux,pfgenyun/tamarin-redux,pnkfelix/tamarin-redux,pfgenyun/tamarin-redux,pfgenyun/tamarin-redux,pnkfelix/tamarin-redux,pnkfelix/tamarin-redux,pfgenyun/tamarin-redux,pnkfelix/tamarin-redux,pnkfelix/tamarin-redux,pfgenyun/tamarin-redux,pnkfelix/tamarin-redux,pfgenyun/tamarin-redux