Add Display and Error impls for proc_macro::LexError
Add Display and Error impls for proc_macro::LexError

This should allow LexError to play much nicer with the `?` operator.

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