Stop markov chain silently failing to complete words

[TehMillhouse]

In most cases, one might want to know if the markov chain didn't manage to accumulate a valid sentence based on a single word seed, so silently ignoring that isn't good practice.

[asmeurer]

This is obviously trivial to fix. Should it raise an exception in this case, or just return the empty string?

[TehMillhouse]

An empty string seems less clean to me, as it's more implicit and less obviously a "problem". Now raises an exception as of 79442c344b5a8e0d004098eff1ece15e6b98ea91