Fixed manually promoting wrap files with a full path, e.g.
`meson wrap promote subprojects/s1/subprojects/projname.wrap`,
which resulted in an error before (new test added:
`./run_unittests.py AllPlatformTests.test_subproject_promotion_wrap`).
Additionally, running promote with an invalid subproject path now fails
properly. Before, it just silently did nothing (added to test:
`./run_unittests.py AllPlatformTests.test_subproject_promotion`).