Allow variant with no #fail attribute

[kngwyu]

If a variant doesn't have fail attribute, use its' name to display.
E.g.

#[derive(Debug, Fail)]
struct StructWithNoAttr;

#[test]
fn struct_with_no_attr() {
    let s = format!("{}", StructWithNoAttr {});
    assert_eq!(&s[..], "StructWithNoAttr");
}

I don't think All variants must have display attribute is user friendly, but feel free to close this PR if you don't like it.

[dekellum]

I like the high level feature concept. But is this not what Debug offers already? Should the Debug::fmt be what is actually used as a fallback for no "display attribute", e.g. or_else(|| format!("{:?}", self)) ?

[kngwyu]

Should the Debug::fmt be what is actually used as a fallback for no "display attribute"

Agreed, I modified to do so.

[kngwyu]

Now I noticed it's a breaking change.
Like https://github.com/rust-lang-nursery/failure/blob/master/failure_derive/tests/no_derive_display.rs , now we can write manual implementation if a struct/enum derives Fail and it has no fail attribute.
So Display implementation can cause conflicts 😓