Distinguish internal errors to provide better error messages

[dpordomingo]

from https://github.com/src-d/sourced-ce/pull/162#discussion_r307696411

disclaimer, I might be wrong because I didn't paid too much attention to the described problem; I just left this as a product of a discussion from https://github.com/src-d/sourced-ce/pull/162

since we're bubbling errors in different places, without distinguishing them, then providing meaningful errors is a difficult task, and then we find strange (or not meaningful) error messages from time to time.

This could be considered with #163

Originally posted by @dpordomingo in https://github.com/src-d/sourced-ce/pull/162