Addresses review comments

Addresses review comments

* rebased
* removed `human` (deferring removing `internal` to a later PR)
* cargo_test.rs - fails on other error kinds
* unnecessary `map_err(CargoError::from)` removed
* fold NetworkError entirely into CargoError
* added justification comment for `extend_lifetime`
* various formatting goofs

The following tests are currently failing:
* `http_auth_offered`
* `custom_build_script_failed`
* `build_deps_for_the_right_arch`
* `dep_with_bad_submodule`
* `update_with_shared_deps`
* `finds_author_email`
* `finds_author_user`
* `finds_author_user_escaped`
* `finds_author_username`
* `finds_git_author`
* `exit_code`