In 022045ae05 a regression was introduced: if no *slideshowAPI*
is specified in the branding file, Calamares refuses to start, with
a YAML failure.
Before the refactoring, we had `YAML::Node doc` and looked up
the *slideshowAPI* in it with `doc["slideshowAPI"]`. After the
refactoring, we had `const YAML::Node& doc`. The `const` makes
all the difference:
- subscripting a non-existent key in a mutable Node silently
returns a Null node (and possibly inserts the key);
- subscripting a non-existent key in a const Node returns an
invalid or undefined node.
Calling IsNull() or IsScalar() on a Null node works: the functions
return a bool. Calling them on an invalid node throws an exception.
So in the **const** case, this code can throws an exception that it
doesn't in the non-const case:
`doc[ "slideshowAPI" ].IsScalar()`
- Massage the code to check for validity before checking for scalar
- Add a `get()` that produces more useful exception types when
looking up an invalid key
- Use `get()` to lookup the slideshow node just once.