Improve `missing EOF at '_ensures'' syntax error

[bafain]

When _ensures is not used to annotate a function like in the following code:

_ensures true

The same applies to the annotations _requires when not used to annotate a function and _invariant when not used to annotate a loop.

[jspam]

That error seems to be shown every time the parser comes across a token it cannot parse in that context. So a general error message such as This statement does not belong here would be sufficient.