Failure to rebind such closures is not necessarily related to them
being created by `ReflectionFunctionAbstract::getClosure()`, so we fix
the error message.
Closes GH-6424.
This prohibits binding closures returned by
ReflectionMethod::getClosure() to a $this, which is not an
instance of the method scope. This restriction was already in
place for internal methods, now it is extended to user methods.
ML discussion: http://markmail.org/message/zepnhdyr3kij6di6
