Change env var getters to error recoverably
Change env var getters to error recoverably

Before this, `std`'s env var getter functions would panic on
receiving certain invalid inputs. This commit makes them
return a `None` or `Err` instead.

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