Less agrressive errors when rendering functions with a single non-numerical arugment

[berekuk]

Right now the output is too frightening and can convince users that it's their fault, while it's really the playground's limitation.

https://www.squiggle-language.com/playground/#code=eNqrVirOyC8PLs3NTSyqVLIqKSpN1QELuaZkluQXwUQy8zJLMhNzggtLM9PTc1KDS4oy89KVrJTSNCo0FWwVKvQSlWoBJj4auw%3D%3D

When we get type inference, we'll be able to handle this better. In the meantime, just a grayed out "This function can't be rendered" (with the ability to expand the error on click?..) would be better.