Tweak error message heuristic for LHS unions

Summary:
Tweak the heuristic as follows, for LHS unions:
- If no candidate is above 40 of score (40 corresponds to the limit of a matching top-level constructor) then produce an `Incompatible` error without further explanation, as before;
- Otherwise, if no candidate matches, then just display an explanation for the one with the lowest score;
- Otherwise, as before: display a partial match.

Reviewed By: ilya-klyuchnikov

Differential Revision: D74998988

fbshipit-source-id: 6f3a71237471f96030e77b0f0f3218dd8d9b3ffb

Author: VLanvin

eqwalizer/src/main/scala/com/whatsapp/eqwalizer/tc/TypeMismatch.scala

@@ -262,8 +262,10 @@ class TypeMismatch(pipelineContext: PipelineContext) {
         val tys1 = util.flattenUnions(ut)
         val details = tys1.map(findMismatch(_, t2, seen))
         val lowest = details.minBy(_.score)
-        if (details.forall(_.mismatch.nonEmpty))
+        if (details.forall(_.score < 40))
           Details(Some(subtype.join(tys1), t2, Incompatible), lowest.score)
+        else if (details.forall(_.mismatch.nonEmpty))
+          lowest
         else {
           lowest.mismatch match {
             case None => lowest