Mark Error impl for LayoutErr as stable.
Mark Error impl for LayoutErr as stable.

This impl was effectively stable. #[unstable] had no effect here,
since both Error and LayoutErr were already stable.

This effectively became stable as soon as LayoutErr became stable, which
was in 1.28.0.

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust