Better error message from rx for len

[jbednar]

Right now, len(x) on a reactive expression gives an error even on objects that do have a length:

from param import rx
l = rx([1,2,3])
print(l.rx.len())
len(l)

TypeError: object of type 'rx' has no len()

I believe we can't implement __len__ reactively, but can't we provide a better error message in this case? E.g. it seems like we could print a warning saying that the value returned won't be reactive (and what to do instead (.rx.len()), and then return the non-reactive length.

I'm not sure if there are any other such methods where we can similarly help guide the rx user towards correct code with less pain.