Uncaught Error when incorrect problem name selected

[anchat1990]

When trying out stream-adventure I found that a typo in the problem name when "select"ing problem leads to the following uncaught error

> 
> var p = adv.fn();
>                ^
> 
> TypeError: Cannot read property 'fn' of undefined
>     at Shop.select (/home/achatterjee/sandbox/stream-adventure/node_modules/adventure/index.js:248:16)
>     at Shop.execute (/home/achatterjee/sandbox/stream-adventure/node_modules/adventure/index.js:97:14)
>     at Object.<anonymous> (/home/achatterjee/sandbox/stream-adventure/bin/cmd.js:18:6)
>     at Module._compile (module.js:409:26)
>     at Object.Module._extensions..js (module.js:416:10)
>     at Module.load (module.js:343:32)
>     at Function.Module._load (module.js:300:12)
>     at Function.Module.runMain (module.js:441:10)
>     at startup (node.js:139:18)
>     at node.js:968:3
> 

I'm about to put in a pull request for this error to be caught and an informative error message to be displayed.