First successful result from parallel calls under a global timeout

A backend API is slow and sometimes fails. To answer quickly, call several of these functions in parallel and return the first result that succeeds. Write a function that takes the list of calls and one overall deadline. Return the first success; if every call fails, report the failures; if the deadline passes first, give up with a timeout. Make sure calls that are still running after you return are cancelled or cleaned up, and talk through how errors and the timeout are surfaced.