Better error messages

In a discussion on IRC last night, it was brought up that our error messages kinda suck sometimes. I tend to agree (merkledag: not found).

To improve this, we can distinguish between a few different failure modes for common commands:

1. Cannot find any providers for the given hash
2. Cannot connect to any providers for the given hash
3. MERKLEDAG: NOT FOUND (or maybe: failed to find hash)

Then, if the daemon is offline, we can return something like:
data not found *locally*.

(for context, i also just read [this article](https://blog.keras.io/user-experience-design-for-apis.html) and enjoyed it)