Inspired by rust-lang/rust#49162 (comment)

With ?-in-main about to stabilize, it would be nice to have an "it just works" return type for main so that one can start a bin with fn main() -> InsertSomethingEasyHere { until you care about doing something else.

Edit: Would also be nice for tests, since they never care about the error type, just whether there was one. (cc rust-lang/rust#48854)

Tagging lang in case this needs tweaks to ? to work well with heterogeneous error types.
Tagging libs in case this should be an alias for something like Result<(), Box<Error>> or involve failure in some way.