Improved Error Handling for Map Viz

[ixipixi]

Is your feature request related to a problem? Please describe.
Given a list of countries plotted on a map, sometimes a country will not show up on the map because the code has a leading or trailing whitespace in it. We could also consider the case where there is a 2 digit code that doesn't correspond to a country code.

Describe the solution you'd like
I could think of a few options for handling this:

• Implementing string cleaning on "country" semantic types (doesn't handle accurately formatted but invalid codes)
• Indicate on the map viz settings that there are some invalid values in the data set (valuable even if it doesn't indicate which ones)
• Introduce a custom expression that would allow users to validate country codes