Emit error for all unset configuration variables

[jquorning]

Improve user experience by emitting error for each unset configuration variable.

Output of alr get lvgl_ada:

error: Configuration variable 'lvgl_ada.density_per_inch' not set and has no default value.
error: Configuration variable 'lvgl_ada.pixel_bit_depth' not set and has no default value.
error: Configuration variable 'lvgl_ada.horizontal_resolution' not set and has no default value.
error: Configuration variable 'lvgl_ada.vertical_resolution' not set and has no default value.
error: Configuration failed

Before:

error: Configuration variable 'lvgl_ada.density_per_inch not set and has no default value.

[Fabien-Chouteau]

@jquorning you might want to add a test case for that.

[mosteo]

I was bitten by this very same case once. Thanks!

[mosteo]

Minor last thing: I would move the test from the get to the config dir, as it applies outside of the get command.

[mosteo]

Thanks, merged.