how to catch A ‘UNIQUE constraint failed’ 404 in django

from django.db import IntegrityError

except IntegrityError:

This is what you need.

EDITED for @mbrochh:

from django.db import IntegrityError

except IntegrityError as e: 
    if 'unique constraint' in e.message: # or e.args[0] from Django 1.10
        #do something

Yes, you can be more precise but in question case UNIQUE failed is highly likely.