Improving procmacro error reporting

[idubrov]

Improving error reporting of procmacro by using compile_error! macro and quote_spanned!.

Let me know what you think.

I haven't done it 100% of the way (changing is_cause would require a bit more of the refactoring).

For example, for the following code:

#[derive(Debug, Fail)]
enum Error {
    #[fail(display = "Hello")]
    Hello,
    Bye,
}

you will get:

error: All variants must have display attribute.
  --> failure_derive/tests/no_derive_display.rs:27:5
   |
27 |     Bye,
   |     ^^^

[davidbarsky]

@idubrov This is a solid PR, thanks! I'm just working on fixing the merge conflict, but I'll merge right after that.

[davidbarsky]

Ah, to support this, we're using a more recent version of Backtrace that doesn't work on Rust 1.18. I'll try to see if Backtrace 0.3.11 or 0.3.12 would build successfully on Rust 1.18, because #281.

[mitsuhiko]

Squash merged in 15b6798