Improve error/help message wording when no country is selected

[sophiamersmann]

Description

If no country is selected, Grapher (sometimes) says 'No matching data' and then in a smaller font size 'Try adding countries and regions to display data'. Users might not understand what 'No matching data' means – as noted by Bobbie.

Note that other charts do have a more specific error message, for example this one

Expected behaviour

We should say 'No [entity types] selected' when appropriate.

While we're at, it might make sense to revise other error/help messages as well.

Steps to reproduce

1. Go to https://ourworldindata.org/grapher/obesity-prevalence-adults-who-gho?tab=chart

Screenshots