In new arch, make Layer operations return Result instead of panicking internally

[hweom]

Instead of

pub trait Layer<B: IBackend>: Debug {
    fn compute_output(&self, backend: &B, context: &mut Context);
    fn compute_gradients(&self, backend: &B, context: &mut Context);
}

we would have

pub trait Layer<B: IBackend>: Debug {
    fn compute_output(&self, backend: &B, context: &mut Context) -> Result<(), LayerError>;
    fn compute_gradients(&self, backend: &B, context: &mut Context) -> Result<(), LayerError>;
}