Instead of resolving a node, we resolve a path. This resolved path
is then re-used for Cat and Ls. This way, a resolve operation is
only done once.

The error messages for a failed resolve is changed from `ipfs cat …`
to `ipfs resolve …` to better reflect the API calls. The test is
updated accordingly.

License: MIT
Signed-off-by: Remco Bloemen <remco@2π.com>