Use rustc_on_unimplemented's trait name argument in try

[estebank]

Follow up to #43000 and #43001. Fix#42694.

[rust-highfive]

r? @brson

(rust_highfive has picked a reviewer for you, use r? to override)

[kennytm]

Travis failed with:

[00:02:28] error[E0230]: there is no type parameter Try on trait Try
[00:02:28] --> /checkout/src/libcore/ops/try.rs:18:1
[00:02:28] |
[00:02:28] 18 | / #[rustc_on_unimplemented = "the `?` operator can only be used in a function that returns `Result` \
[00:02:28] 19 | | (or another type that implements `{Try}`)"]
[00:02:28] | |_______________________________________________________________________^

This probably needs a cfg_attr(not(stage0), ...).

[estebank]

@kennytm@brson, I'm thinking that we should just wait until #43000 is part of the release. It's a minor thing, I feel.

[arielb1]

Yeah that will happen the next time we'll have a snapshot, which should be in a ~week or so

[alexcrichton]

@estebank I think we've got a new stage0 so this should be ready for a rebase!

[estebank]

@alexcrichton This PR is ready for merging.

[alexcrichton]

@bors: r+

[bors]

📌 Commit 4f7834e has been approved by alexcrichton

[bors]

⌛ Testing commit 4f7834e with merge 4d5150c...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 4d5150c to master...