docs: Added throws PHPDoc tags in ISO3166

[florentausha]

This PR adds @throws PHPDoc tags in file ISO3166.php.
This improvement makes the library easier to discover.
Thanks to this, the user doesn't have to look into the source code to know how this lib works.

[alcohol]

Should exactName also be documented in that case?

[florentausha]

Thanks, I did not see this function!

It has been added :)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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