Conflicting issues for a trait error should name the trait

[jdm]

It displays the traits in question, but if they're generated by macros you can't tell what the problem is.
/home/jdm/sdb/servo/src/components/script/dom/bindings/element.rs:292:4: 297:5 error: conflicting implementations for a trait

It shouldn't be hard to throw a name in here.