Better desugaring failures when boxing fails

The new type-vs-constraint patch (!8750 (merged)), when it lands, allows us to do things like

[ I# (x2 +# y2) | I# x <- xs, let x2 = x +# 1 | I# y <- ys, let y2 = y -# 1 ]

This is a parallel list comprehension where we want to collect a bunch of pairs (x2,y2), but we can't make a pair of unboxed values. So we box them before building the pair.

• This is done in GHC.Core.Make.wrapBox and friends.
• See also Note [Boxing constructors] in GHC.Builtin.Types

But

• We can't (currently) guarantee to do this - notably for nested unboxed tuples.
• If we fail to box, GHC crashes

But really we should make the desugarer stop with a civilised error message.

Should not be hard, but requires some careful attention to the various call sites of mkBigCoreTup etc.

Closely related ticket: #21165 (closed)