add the annotation read so far to error messages.

This requires https://github.com/doctrine/lexer/pull/3 and prints the annotation as read so far for a much more useful error reporting. Example:

`@AnnotatedDescription(desc = "foo", foo = "bar", )`

produces:

```
[Syntax Error] Expected PlainValue, got ')' at position 49 in class AnnotationDemo.
Annotation read so far:

@AnnotatedDescription(desc = "foo", foo = "bar", 
```