Check for _kirigamiTheme override in Units, improve plugin error message

Adds a check for _kirigamiTheme, as we do the same in PlatformTheme::qmlAttachedProperties.

Also when failing to find the platform plugin, print the style name:

kf.kirigami.platform: Failed to find a Kirigami platform plugin for style "Fusion"

Usually this makes it crystal clear why the platform plugin failed to be found, like if the style is a built-in Qt style instead of one of our own.