Use std::process::ExitCode's SUCCESS and FAILURE

[lelandjansen]

exit_code.rs provides various exit codes such as SUCCESS, FAILURE, USAGE, OSERR, CONFIG, etc. specified in /usr/include/sysexits.h. We should replace exit_code::SUCCESS and exit_code::FAILURE with std::process::ExitCode::SUCCESS and std::process::ExitCode::FAILURE, respectively, when the API stabilizes.

API docs: https://doc.rust-lang.org/std/process/struct.ExitCode.html
PR: rust-lang/rust#48711

Introduced by #13.