Clarify Error description and detail should be single line

[stepancheg]

When description and detail are known to be single line, they can
be used like this, and programmer can be sure that message is
formatted nicely.

println!("could not download file: {}, trying again in 5s", e.description());

[aturon]

@stepancheg Thanks for the PR! It does seem like a good idea to standardize on a convention here. I believe this is already followed within IO errors. But I'd like to get a little bit of community feedback before landing this change.

Would you mind either posting this proposal to http://discuss.rust-lang.org/ or as a short conventions RFC?

Thanks again.

[stepancheg]

Here is it: http://discuss.rust-lang.org/t/conventions-and-enhancements-of-error-trait/779

[bstrie]

I think we should hold off on work like this until we get a chance to do a comprehensive pass on error-handling conventions, rather than working piecemeal on something that may get thrown out entirely for all we know. I know Armin at least thinks that Error might be the wrong way to go in the long term.

[steveklabnik]

There has been no discussion on Discourse, and there's been some recent talk about changing errors from @mitsuhiko. What should we do with this PR?

[alexcrichton]

For now I'm going to close this in favor of rust-lang/rfcs#492 to help clear out the queue and take some action on this. Depending on the resolution of that RFC we can reopen this PR.