Skip user-condition validation when the auth table is missing

[arpitjain099]

Fixes #96.

validate_user does UserModel.objects.get(...). That validator runs inside the flag_conditions_check system check, which only catches ValidationError. On a fresh database the system checks run before the auth tables are created (for example during the first migrate), so the lookup raises OperationalError / ProgrammingError, and since the check does not catch it, the error propagates and aborts the migration. The maintainer confirmed this in the thread, and a commenter suggested catching the database error.

This catches OperationalError / ProgrammingError in validate_user and skips validation in that case. The username cannot be checked when its table does not exist yet, and it is validated normally once the tables are in place, so the only behavior change is that a missing table no longer breaks migrate. I went with skipping rather than raising a ValidationError, since raising would surface a misleading "invalid value" warning on every migrate when the value is actually fine.

Added test_user_table_missing_is_skipped, which mocks the manager lookup to raise each error and asserts validate_user does not raise. Full flags.tests.test_conditions_validators suite passes (13 tests).