### Description

A query like `from employees | where true` is perfectly valid. So is `from employees | where still_hired`. In other words, a simple boolean expression is accepted, even if that expression is a field. For fields `where [field_name]` is the short version of `where [field_name] == true`.

But, for fields that are not booleans `from employees | where salary` for example, ESQL throws an exception:

```
                "type": "class_cast_exception",
                "reason": "class org.elasticsearch.compute.data.IntVectorBlock cannot be cast to class org.elasticsearch.compute.data.BooleanBlock (org.elasticsearch.compute.data.IntVectorBlock and org.elasticsearch.compute.data.BooleanBlock are in unnamed module of loader java.net.FactoryURLClassLoader @c6a692e)"
```

The error message should be more user-friendly, like validating the parameters of regular functions.
x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/analysis/Verifier.java
var exp = e instanceof Alias ? ((Alias) e).child() : e;
                    if (exp instanceof AggregateFunction aggFunc) {
                        Expression field = aggFunc.field();
                        
                        if ((field instanceof FieldAttribute
                            || field instanceof MetadataAttribute
                            || field instanceof ReferenceAttribute
                            || field instanceof Literal) == false) {
                            failures.add(
                                fail(
                                    e,
                                    "aggregate function's field must be an attribute or literal; found ["
                                        + field.sourceText()
                                        + "] of type ["
                                        + field.nodeName()
                                        + "]"
                                )
                            );
                        }
                    } else if (agg.groupings().contains(exp) == false) { 
                        failures.add(
                            fail(
                                exp,
                                "expected an aggregate function or group but got ["
                                    + exp.sourceText()
                                    + "] of type ["
                                    + exp.nodeName()
                                    + "]"
                            )
                        );