Add a process.beforeExit / process.exit handler that checks if a task is currently running

[Arnavion]

... and fails the build if one is found with process.exit(1) etc..

It would help detect async tasks that failed to call complete(). Currently, this would result in node exiting with code 0 because there is no more pending code to execute, which gives the false impression that the build succeeded.

For example, TypeScript hit this in this PR because one of the async tasks failed to call complete()