This improves error messages for `a |> b(x)`.
We need to do a special check when looping over the args
and unifying. This information is within a function that does not belong
to pipe typer so I used a closure to forward along a way to add
metadata to the error when the first argument in the loop has a
unification error. Simply adding the metadata at the pipe typer
level is not good enough because then we may annotate regular
unification errors from the args.
135dbd8335