Warn and exit when running inside tmux on OS X

tmux loses access to its original namespace on OS X which causes Chrome to
crash.

See https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard#interaction-with-tmux
and http://savanne.be/804-running-karma-and-chrome-under-tmux-on-osx

Can be reproduced with the Google Music app. Chrome will then display the "Aw snap" crashed tab message.