Nicer error messages: unifyT should be Either

[amosr]

At the moment unifyT for unifying source types returns a Maybe Subst.
It should return an Either Err Subst so we can differentiate errors.
There are at least two error types:

• occurs check / recursive type
• cannot unify
  but we might be able to split them up further.

https://github.com/ambiata/icicle/blob/d86f4846612fed788ace4fc02dc5748e3e6fe2b8/src/Icicle/Source/Type/Subst.hs#L156