Encapsulate BacktraceFrame in a local type

[brson]

trafficstars

The backtrace crate is < 1.0 so error-chain can't hit 1.0 without either bumping backtrace or encapsulating it.

It looks like the out-of-crate Backtrace type is already encapsulated, but BacktraceFrame is still exposed.

I'm kinda lukewarm on this. With all the changes being requested in error-chain, who knows if it'll actually hit 1.0 any time soon anyway, and it's just duplicating what's in the backtrace crate, basically making stability promises on behalf of that crate.

[Yamakaky]

Let's just make this issue low priority.