Provide a warning message layer

[julianna-langston]

Example situation
A visual chart shows a scatter plot and a violin plot. While chart2music supports scatter plots, it doesn't support violin plots. I want to make the scatter plot accessible to users, but I don't want to hide the fact that there's a violin plot also present in the visuals.

Request
There should be a way to communicate inaccessible layers. Maybe the user is told that there are 2 layers. They go to the first one, a scatter plot, and can navigate as normal. They navigate to the second layer, like "Chart contains unsupported chart type: violin plot.", perhaps optionally with some kind of alt text.

[julianna-langston]

Addressed with #282