Add error constants

[dagargo]

Since the project beginning, errno values have been used as a return value in many functions because the connector API works as a filesystem OS layer. However, it is probably better that the use of these values is entirely removed.

The idea is to have a small set of error constants to map somehow the used errno values.

Thanks to this, it would be possible to have uploads being truncated and notify the user about this special case but it might be useful in other cases too.

[dagargo]

While no constants have been added, I've pushed a few commits to the CLI to remove uses of errno values. See 0fbdf1c, 8356e77 and e961b95.

This only leaves the connectors code to be examined but I'm not sure if I'll ever change that because I don't want to break anything. Still, I'm leaving this open for now.