Traditionally you write code which checks the Widget to see if it's Gonzo and if not you throw an exception. Callers can pick, for this function in particular, whether to handle the Exception, in which case they get that Exception to look at, or they can "bubble it up" to be handled in their caller, and so on all the way to the top of the program where if it bubbles up it's reported and then exits the program.

With Contracts you write a contract for the function with a pre-condition that the Widget is Gonzo. Your users (programmers who might call doodle_widget) can pick: If they fail a contract the program exits immediately reporting a violation ("quick enforce"), it reports the violation via a global contract handler and then exits ("enforce") or it just reports to the handler but doesn't exit ("observe") or finally, they ignore it entirely ("ignore")

These just aren't that different. The contract is maybe slightly better because of the enhanced semantic discovery - you could imagine tooling which gives you a yellow squiggly line because your code violates a contract requirement for example, it's definitely not practical to check exception raising that way.