Improve fallback reporting for aggregates

### What is the problem the feature request solves?

If incompatible casts are not enabled, I see the following fallback message in `spark/src/test/resources/tpcds-plan-stability/approved-plans-v1_4-spark4_0/q92.native_iceberg_compat/extended.txt` when running the plan stability suite.

The fallback message does not suggest enabling incompatible casts, so is not very helpful.

```
+- HashAggregate [COMET: Unsupported result expressions found in: List((1.3 * cast((avg(UnscaledValue(ws_ext_discount_amt#1))#2 / 100.0) as decimal(11,6))) AS (1.3 * avg(ws_ext_discount_amt))#3, ws_item_sk#4)]
```

### Describe the potential solution

_No response_

### Additional context

_No response_