add descriptive errors for var(..)

[clifton]

adds context to error generated by dotenv::var("MISSING_VAR")

before

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: EnvVar(NotPresent)'

to

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: EnvVar(NotPresent): MISSING_VAR'

[sonro]

@clifton if you still consider this change necessary, please contribute to the discussion #25.

[allan2]

Closing because I don't see this as the final format of the error API.

Feel free to contribute in the discussions.