Error and .unwrap() do not agree on traits

[maurer]

Error currently requires Display, while .unwrap() requires Debug.

I'm unsure of what each should require, but I think they should agree so that it is possible to have code which returns Result<_, Box<Error>> (which is useful to allow try! across functions returning different types of error codes), and be able to .unwrap() the results.

[Stebalien]

There was a lengthy discussion about this towards the bottom of the RFC: rust-lang/rfcs#565.

[steveklabnik]

/cc @aturon , I vaguely remember this might change? If not, then this should be in the RFCs repo.

[aturon]

Nominating for 1.0-beta P-backcompat-libs.

(The Error trait isn't #[stable] yet, but it probably will be for 1.0).

[pnkfelix]

1.0 beta, P-backcompat-libs, I-needs-decision.