Better errors for predicate parsing

Right now every problem in a `pre`, `post`, or `require` block gives the very unhelpful message:

```
could not parse predicate
```

We should give more information about what failed in each case (unsupported operation, reference to a variable which is not argument, and so on).

Start by locating the predicate parsing code and the existing "could not parse predicate" message for pre, post, and require blocks. Inspect how unsupported operations and non-argument variable references are handled; done means each failure reports useful information about the specific problem.

Written by the indexing model from the issue text.