Simplify EdgeGatewayInterface defenses

EdgeGatewayInterface is an internal-use class, we don't need
to be excessive with the input checking.

Simplified the input checking to ensure that we don't unnecessarily
create NilClass exceptions.

Tests adjusted tests accordingly.
Merge pull request #42 from alphagov/improve_bad_input_handling_clarity

[#70437918] Simplify EdgeGatewayInterface defenses