report smartctl failure codes to the user

as it turns out there are a number of issues that do not result in an actually bad SMART status and would not get reported to the user. to keep the code lean we'll simply look at the exit code bits of smartctl as they more or less cover all failure scenarios anyway. we'll then translate those to pretty strings and expose them on the Device objects for consumption in the qml KCM as "instabilities". they are set visually apart from actual bad status through different icon and description because it's hard to say if an instability is in fact indicative of imminent hard-failure or merely a hiccup (e.g. power loss during a disk operation).

BUG: 429804 FIXED-in: 5.22