Suggest .err().expect() if .expect_err() is not implemented

[lukaslueg]

If T in Result<T, _> is not fmt::Debug, the following will fail to compile. The compiler rightfully blames fmt::Debug not being implemented for Result<Foo, ()>.

struct Foo;

Result::<Foo, _>::Err(()).expect_err("Foo!");

However, users may get stuck if they can't - for whatever reason - add fmt::Debug. A simple workaround is

Result::<Foo, _>::Err(()).err().expect("Foo!");

which has a somewhat degraded error message in the panic case, yet does what it's being asked for. Maybe the compiler could suggest that?