Return descriptive error codes from e2e tests

[prajakta-gokhale]

Description

The e2e tests in this package return a numeric return code of 0 for success and 1 otherwise. This does not help identify the cause of build failures easily, without looking at the long logs produced.

Refactor the return code part in system_metrics_e2e_test and topic_statistics_e2e_test to make the failure code less generic and more descriptive of the cause of failure.

_Originally posted by @dabonnie in https://github.com/ros-tooling/system_metrics_collector/pull/125/files_

Test Plan

• Manual testing of new return codes

Documentation Plan

• Document the return codes in e2e tests

Release Plan

• Check in PR

Acceptance Criteria

• [ ] Code has been implemented, reviewed and merged.
• [ ] Test plan has been completed
• [ ] Release plan has been completed

Once all above items are checked, this story can be moved to done.

Resources

• system_metrics_e2e_test
• topic_statistics_e2e_test

[dabonnie]

Some context why descriptive return codes are useful: https://github.com/ros-tooling/action-ros-ci/issues/143