Check constructor argument types in Messengers and handlers

[eb8680]

In this topic on the forum, a user passed a mask tensor to poutine.mask as an argument poutine.mask(mask) rather than a keyword argument poutine.mask(mask=mask), attempted to use the resulting Messenger as a context manager, and got an obscure error message as a result:

ValueError: Expected mask to be a boolean but got <class ‘NoneType’>

It would be good to raise more informative error messages in such cases that would make the issue clear (in this case, raising a TypeError or ValueError when initializing the Messenger's fn attribute to something clearly not callable).