Improve error message for unexpected ftl:export

[matt2e]

To export a verb you write this code in go:

//ftl:verb export
func ...

We recently added the ftl:export directive for FSMs:

//ftl:export
fsm := ...

It would be easy for devs to get confused about how to make a verb exported and they may easily make the mistake of using ftl:export on a verb.
When doing so we get this error:
unexpected directive *compile.directiveExport

We should come up with a better error message explaining what they need to do to resolve the issue.

Also, do this for other declarations that can be exported but not via //ftl:export, eg: data