RFC Mentoring Opportunity: Permit `?` in `main()`

[nikomatsakis]

@withoutboats

The idea I like is defining a new trait and having main be allowed to return any type that implements that trait.

Agreed.

@withoutboats

std could define it for () and Result<(), Box<Error>>.

Probably Result<(), T> where T: Error but yes.

@withoutboats

Adding the Fallible alias seems like a fine idea but orthogonal to this change.

I agree it's orthogonal but it seems important to reaching the final goal of pretty examples on the front page. =) Without it though we can at least get examples that show off best practices, I guess.