Fail fast on missing methods in state object

[chrisroos]

This supersedes PR #2071

The SmartAnswer::State object is an OpenStruct which means that it responds to undefined methods with nil. This can lead to hard to find bugs in next_node blocks (particularly larger ones, e.g. this one in pay-leave-for-parents) as a simple typo will mean that the program runs but doesn't return the desired result.

The main change in this PR overrides SmartAnswer::State#method_missing so that it raises a NoMethodError exception if we try to call a method that's not previously been defined.

# Behaviour before this change
>> state = SmartAnswer::State.new(start_node = nil)
>> state.foo
=> nil
>> state.foo = 'bar'
>> state.foo
=> 'bar'

# Behaviour after this change
>> state = SmartAnswer::State.new(start_node = nil)
>> state.foo
=> NoMethodError: undefined method 'foo' for SmartAnswer::State
>> state.foo = 'bar'
>> state.foo
=> 'bar'

I've also updated all Smart Answer flows where the code was relying on the old behaviour of the State object, by explicitly setting the variables to nil that are used later in the flow.

I started out with the commit titled "Implement State#method_missing" as the first commit in order to identify the flows that needed updating. I moved it to become the final commit once I was happy that I'd updated all flows.