Event API: getField should throw an exception when field is missing

Make getField throw exception if the requested field is missing

Proposal:

```
class FieldMissingException extends Exception { /* ... */ }

Object getField(...) throws FieldMissingException
```

Start by locating the Java Event API implementation of getField and any tests covering missing fields. Read how existing API exceptions are defined and used; done means a missing requested field consistently raises the proposed checked exception and the relevant tests document that behavior.

Written by the indexing model from the issue text.