Improve suggestions for inexhaustive case expression error

[GearsDatapacks]

Closes #3544.
Instead of just returning Failure when there are no rows, it runs exhaustiveness checking on the first subject of the case expression to give better suggestions to improve it.
In future we should maybe check other subjects too, to further improve this. Although this could produce a big decision tree, and we don't do that at the moment for other patterns so I left it.
For example, at the moment, for this code:

case True, False {}

It will suggest True, _ and False, _. (Actually it won't, because of #2426, but that's a separate issue)

However, if we check both subjects, it will suggest all four possible combinations.
However, consider this possibility:

type Wibble { A B C D }
case A, B, C, D {}

This would produce 4^4 = 256 possible options, which is a bit much. So I've left it with just the one for now.

[lpil]

Oh this is lovely work, thank you very much! Left some small notes below

[GearsDatapacks]

Alright, I've addressed all of the points you raised.

[lpil]

Thank you!!!