auto merge of #16130 : apoelstra/rust/decode-error, r=alexcrichton

A quick and dirty fix for #15036 until we get serious decoder reform.

Right now it is impossible for a `Decodable` to signal a decode error, for example if it has only finitely many allowed values, is a string which must be encoded a certain way, needs a valid checksum, etc. For example in the `libuuid` implementation of `Decodable` an `Option` is unwrapped, meaning that a decode of a malformed UUID will cause the task to fail.