Add methods for inspecting JoinErrors

[dfoxfranke]

Motivation

Currently there's no way for a program to get information about a JoinError except to except to scrape its Display or Debug implementations. And even though the JoinError retains the payload
of a panic, there's no way at all to do anything with it since the Display and Debug implementations don't show it.

Solution

This adds is_cancelled() and is_panic() methods to JoinError, as well as an into_panic() method which, when applicable, returns the payload of the panic.

[carllerche]

Thoughts @seanmonstar? I know we hit some problems w/ Sync before... do we want to expose the boxed error?

[dfoxfranke]

Ping. What are your intentions for this PR?

[carllerche]

Thanks 👍 I made tweaks, primarily into_* should not return Result. I switched it to panic and added a try_into_* conversion.

[udoprog]

LGTM