Improved Error Handling for Map Viz

Nobody has claimed this yet.

**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

No files, tests, or entry points are named. Start by locating the map visualization handling for country semantic types and review how invalid or whitespace-padded country codes are treated. The issue needs a decision among the proposed approaches before completion can be defined.

Written by the indexing model from the issue text.