Use a special fallback skin when selected skin is unavailable

It just displays a helpful message that explains why and how to
install and enable skins. There is no navigation nor other basic page
elements (like the logo or site notice), since this is not intended to
be a fully functional skin.

Bug: 68332
Change-Id: Id14fbb8733cd8fbb912a724ac658f5e7244364b5
Merge "Use a special fallback skin when selected skin is unavailable"