Remove unreachable! from macros.rs
Remove unreachable! from macros.rs

replaced unreachable! with error handling using Option.

Closes #2558

rust-lang/rustfmt,graydon/rust,graydon/rust,rust-lang/rustfmt,aidancully/rust,SingingTree/rustfmt,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,SingingTree/rustfmt,graydon/rust