"Decodable with error probability less than $\varepsilon$" is tautological to "max-a-posteriori decoder $\arg\max_{\text{message}}P(\text{message}|\text{observation})$ fails with probability less than $\varepsilon$." By the problem's definition no decoder can have error probability less than this one.
It is useful to notice from the Bayes rule formula that when all codewords are equally likely, the max-a-posteriori decoding is the same as the max likelihood decoding.