Refer to the bash_init.sh script to understand how commands can change their behavior based on return code.
The textmate dialog usage can be reverse engineered from [this bundle support Ruby script] (https://github.com/textmate/bundle-support.tmbundle/blob/d400d9d6a6234ccf3388d185c178b18a29078ada/Support/shared/lib/ui.rb).
Under the covers, a process called tm_dialog2 is being used to open a specific NIB file with an Apple property list. A property list is also returned to indicate updated values in the NIB.
To understand more about the dialog system, read this article: http://blog.macromates.com/2006/new-dialog-system-for-commands/
This is an amazing write up for understanding how the code completion UI works: http://assoc.tumblr.com/post/79108106701/get-started-writing-textmate-2-bundles