prefer reporting errors over warnings

[setharnold]

Hello,

When eval sees a request like: eval: static X: u8 = [1,2][4]; it will report the warning from rustc rather than reporting the error that prevented compilation:

<panicbit> eval: static X: u8 = [1,2][4];
-eval:#rust-offtopic- warning: static item is never used: `X`
-eval:#rust-offtopic-  --> src/main.rs:3:9
-eval:#rust-offtopic- ~~~ Output truncated; full output at 
          https://play.rust-lang.org/?gist=41c374e4777addb6aa7f8e239fbed2f3&version=stable&mode=debug

I think the output would be more useful if it reported the first error, rather than whatever happened to be the first output:

error[E0080]: constant evaluation error

Thanks

[panicbit]

It would be useful to be able to get the compiler's json output from the playground to avoid this becoming a mess.