Failing to `find` a Country shouldn't trigger an exception

[tmtmtmtm]

Everypolitician::Country.find('Narnia') should simply return nil, not throw an Exception.

[tmtmtmtm]

@chrismytton want to persuade us that this should throw an Exception, or are we good to make this change?

[chrismytton]

No objections from me! It's probably worth making the same change for legislatures while doing this, as it's essentially the same.