Use union types in the return type of functions in the error case

Fixes #6663

src/compiler/checker.ts

@@ -10390,7 +10390,8 @@ namespace ts {
                     }
                     else {
                         error(func, Diagnostics.No_best_common_type_exists_among_return_expressions);
-                        return unknownType;
+                        // Defer to unioning the return types so we get a) downstream errors earlier and b) better Salsa experience
+                        return getUnionType(types);
                     }
                 }