Add WeightedError

[sicking]

This fixes #535. I didn't add a impl From<WeightedError> for Error { .. } since I'm not sure how that's used, and so didn't know how to write/test an implementation. Happy to add one if anyone has pointers.

It's a little irky to have choose and choose_weighted indicate errors in different ways (one through Option and one through Result). However negative weights are substantially different than anything that choose has to deal with, so it does make sense.

[dhardy]

Okay, looks good to me. Yes, there is logic in returning different types of errors.

Oh, Error::description is "soft deprecated"? Strange. I'm not too against updating the min Rustc version but it should be easy enough to support 1.22 for now.

[sicking]

Didn't realize that Error::description deprecation had happened since 1.22. Definitely not a reason to update rustc for that.

[sicking]

Pushed an updated PR that should work on 1.22.

[sicking]

Err... I have no idea what's going on. Current builds are failing locally for me due to being unable to find alloc::collections::BTreeMap. Which makes sense since per https://doc.rust-lang.org/alloc/btree_map/struct.BTreeMap.html BTreeMap lives in alloc::btree_map::BTreeMap.

However fixing that in this PR causes test failures with Could not find "btree_map" in "alloc".

[sicking]

Seems to be working now.

[vks]

Some (minimal) docs would be nice.

[dhardy]

I'm surprised there wasn't an error about this actually; we use deny(missing_docs). But it doesn't look too bad:
https://rust-lang-nursery.github.io/rand/rand/distributions/enum.WeightedError.html

[vks]

Yes, this is weird. In any case, I opened #550 to fix this.

[sicking]

The reason it wasn't an error is because we have #[doc(hidden)] for a bunch of stuff in distributions/mod.rs. They are removed in #548 though.