Raise an error when setting attributes on a reactive object

[Maxime Liquet]

When experimenting with the reactive API, I've already done enough the mistake of setting value on the reactive object itself instead of using .rx.value to ask for this raising an error, e.g.:

namex = rx('bob')
namex.value = 'bill'  # wrong but now error, I wanted to type `namex.rx.value`

Actually, I can set any attribute on a reactive object, namex.foo = 'foo' doesn't raise an error.