Better error messages when .train() hasn't been called on BayesClassifier

[tlack]

If one forgets to call .train() on their classifier, here is the error you get from .classify():

> b.classify('dogs');
TypeError: Cannot read property 'label' of undefined
at [object Object].classify (/home/tlack/apps/classifier/node_modules/natural/node_modules/apparatus/lib/apparatus/classifier/classifier.js:37:51)
at [object Object].classify     (/home/tlack/apps/classifier/node_modules/natural/lib/natural/classifiers/classifier.js:84:28)

This is a pretty common mistake; it might be nice to throw an exception in this case instead of just bombing out.

[jdubie]

+1

[chrisumbel]

I'll see about getting this taken care of over the weekend.

[adamramadhan]

ah i see i think this fixed the issue https://github.com/NaturalNode/natural/issues/94

[kkoch986]

so this issue is actually in the apparatus code (line 37) It happens when getClassifications returns an empty array (should only be when the classifier hasn't been trained).

I see two possible ways to improve this:

1. Just return null if no classifications come back. Or,
2. Throw an exception if no classifications come back, alerting you to the fact that the classifier has likely not been trained.

Personally, I'm leaning towards 2.

[kkoch986]

So I've fixed this over in the apparatus code It should now throw an exception if the classifier is not trained.