RFC: error conventions and sugar

This RFC sets out a vision for refining error handling in Rust, in two
parts:

* A stronger set of conventions around `fail!` and `Result`
* Sugar to make working with `Result` highly ergonomic

In a nutshell, the proposal is to isolate uses of `fail!` to an extremely small
set of well-known methods (and assertion violations), with all other kinds of
errors going through `Result` for maximal flexibility. Since the main downside
of taking this extreme position is ergonomics, the RFC also proposes notation
for consuming `Result`s:

* Change macro invocation syntax from `macro_name!` to `@macro_name`.
* Use `foo!` for today's `foo.unwrap()`
* Use `foo?` for today's `try!(foo)`

While the two parts of this proposal reinforce each other, it's possible to
consider each of them separately.

[Rendered](https://github.com/aturon/rfcs/blob/error-conventions/active/0000-error-conventions-and-sugar.md)
I disagree that macros are any more "dangerous" than any other user feature. Dangerous to me is a fairly provocative term that implies unsafety. Perhaps you mean "non-obvious"? In that case, I don't see why `@println` is any better or worse than `println!` as a marker.

As far as the `@` syntax in patterns, I think any ambiguity there is easily resolved by saying that `@foo` is a specific token, and hence spaces can resolve the ambiguity. (We did do a bikeshed some time back and *completely fail* to find an appealing alternative to `@`.) Anyway, `@` is a marginal feature, so I would not allow this concern to derail the larger proposal by itself.