Resolves#3041
## Description:
- Add a test to ensure an error is thrown when manifest.json specifies a
non-existent flag.
- Fix the underlying issue by removing the invalid flag specification
(see error below).
```
resources/maps/straitofgibraltar/manifest.json -> nations[0].flag "Rif" does not exist in resources/flags
resources/maps/straitofgibraltar/manifest.json -> nations[5].flag "Shilha" does not exist in resources/flags
resources/maps/straitofgibraltar/manifest.json -> nations[6].flag "Andalusia" does not exist in resources/flags
resources/maps/italia/manifest.json -> nations[0].flag "custom:Kingdom of the Two Sicilies" does not exist in resources/flags
resources/maps/italia/manifest.json -> nations[3].flag "custom:Tuscany" does not exist in resources/flags
resources/maps/italia/manifest.json -> nations[5].flag "custom:Modena" does not exist in resources/flags
resources/maps/italia/manifest.json -> nations[6].flag "custom:Parma" does not exist in resources/flags
resources/maps/italia/manifest.json -> nations[8].flag "custom:Kingdom of Sardinia" does not exist in resources/flags
resources/maps/italia/manifest.json -> nations[11].flag "custom:Ottoman Empire2" does not exist in resources/flags
resources/maps/britannia/manifest.json -> nations[19].flag "gb-nir" does not exist in resources/flags
resources/maps/montreal/manifest.json -> nations[0].flag "quebec" does not exist in resources/flags
resources/maps/montreal/manifest.json -> nations[1].flag "quebec" does not exist in resources/flags
resources/maps/montreal/manifest.json -> nations[2].flag "quebec" does not exist in resources/flags
resources/maps/montreal/manifest.json -> nations[4].flag "quebec" does not exist in resources/flags
resources/maps/montreal/manifest.json -> nations[5].flag "quebec" does not exist in resources/flags
resources/maps/montreal/manifest.json -> nations[6].flag "quebec" does not exist in resources/flags
resources/maps/montreal/manifest.json -> nations[7].flag "quebec" does not exist in resources/flags
resources/maps/montreal/manifest.json -> nations[8].flag "quebec" does not exist in resources/flags
resources/maps/montreal/manifest.json -> nations[9].flag "quebec" does not exist in resources/flags
resources/maps/montreal/manifest.json -> nations[10].flag "quebec" does not exist in resources/flags
resources/maps/montreal/manifest.json -> nations[11].flag "quebec" does not exist in resources/flags
```
## Please complete the following:
- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
## Please put your Discord username so you can be contacted if a bug or
regression is found:
aotumuri
Aotumuri