fix: return 400/501 instead of 500 for invalid country codes

[aditya047-stack]

Fixes #83

Changes

• Add not_implemented_501() helper in models/responses.py
• Fix /subdivisions to return 501 for unsupported countries
• Fix /data, /flag, /name, /officialname to return 400 for invalid codes
• Separate ValueError and NotImplementedError handling

##Before

##After

[animator]

It should be data not found error instead of server side error.

[aditya047-stack]

Thanks for addressing this issue and helping to solve it .i have updated the fix based on feedback — changed 400 to 404
for invalid country codes as it's a "data not found"
error rather than a bad request. Looking forward to such help again!