Some user_asserts do not provide a message #9405

These result in compile errors that just say "Error:". Super helpful. The specific example I hit is:

https://github.com/halide/Halide/blob/main/src/Generator.h#L2539

I think we maybe just want to delete that specific assert (why can't I clobber the Func associated with a generator output?), but git grep ' user_assert.*;' | grep -v '<<' reveals many other cases.