Bug 1023179  - Fix FTU video unloading/loading, r=fcampo

See original commit which got backed out: https://github.com/mozilla-b2g/gaia/commit/82cccd171556baa330a2ae26aaf994987aee19c2

Changed _loadMedia logic to always resolve its promise, as we want to complete Tutorial initialization if for some reason an image or video doesnt load. Added unit test for this case.