Improve absolute path warnings

[mvz]

Summary

Aruba warns when its API methods are called with an absolute path, but this warning is not very informative.

Expected Behavior

When the user calls an Aruba API method with an absolute path, Aruba reports the file and line of the user code or cucumber step, so the user can fix it.

Current Behavior

Aruba complains about expand_path being called, even if that happens through several steps inside Aruba's implementation.

Steps to Reproduce (for bugs)

See https://github.com/colszowka/simplecov/blob/master/features/config_coverage_dir.feature#L22-L33 for an example of a scenario using an absolute file name.

Context & Motivation

See #687 (comment).