Our errors for function evaluation can be difficult to understand

**Motivation**

In [this ticket](https://github.com/citusdata/citus/issues/1380) a user was given the error

`ERROR: STABLE functions used in UPDATE queries cannot be called with column references`

while trying to run

```
SELECT master_modify_multiple_shards('UPDATE geonames.geonames
SET geometry = ST_PointFromText(''POINT('' || longitude || '' '' || latitude || '')''
');
```

and quite reasonably wondered which function was `STABLE`. Since implicit casts are often `STABLE` this feels like it could be a common problem.

**Proposal**

We should add a `HINT` which names the offending function.