Document that Error::source() types can not be depended on

[seanmonstar]

All sources/causes in hyper::Error should be wrapped in a private Opaque type that still prints (impl Debug and impl Display) the message, but makes the type no longer leak through err.downcast::<Blah>().

[sfackler]

Is it not going to expose sources either? Since the opaque wrapping can only happen at the first level

[seanmonstar]

I suppose it "couldn't" expose the next source, since it can't wrap a reference, and exposing it would potentially include unstable dependencies... If the second option in #2844 happens, the unexposed sources would still be printable at least.

[seanmonstar]

I filed a proposal for the error handling WG, since that could affect the formatting needed for an "opaque" type.