[Bug] Error message format

[Aurélien Gasser]

On master (c49213ad02fd68a9a9137e63502e6a8f49faf129)

$ substra add objective ./objective
Requests error status 400: {"message":[{"metrics":["Ensure your archive contains a Dockerfile."]}],"pkhash":"f643cad518e8540b9a29b86a75a1400a120e90f050167c4238c275367560c622"}
Error: Could not add objective,the server returned the following errors:
- message: {'metrics': ['Ensure your archive contains a Dockerfile.']}
- pkhash: f
- pkhash: 6
- pkhash: 4
- pkhash: 3
- pkhash: c
- pkhash: a
- pkhash: d
- pkhash: 5
- pkhash: 1
- pkhash: 8
- pkhash: e
- pkhash: 8
- pkhash: 5
- pkhash: 4
- pkhash: 0
- pkhash: b
- pkhash: 9
- pkhash: a
- pkhash: 2
- pkhash: 9
- pkhash: b
- pkhash: 8
- pkhash: 6
- pkhash: a
- pkhash: 7
- pkhash: 5
- pkhash: a
- pkhash: 1
- pkhash: 4
- pkhash: 0
- pkhash: 0
- pkhash: a
- pkhash: 1
- pkhash: 2
- pkhash: 0
- pkhash: e
- pkhash: 9
- pkhash: 0
- pkhash: f
- pkhash: 0
- pkhash: 5
- pkhash: 0
- pkhash: 1
- pkhash: 6
- pkhash: 7
- pkhash: c
- pkhash: 4
- pkhash: 2
- pkhash: 3
- pkhash: 8
- pkhash: c
- pkhash: 2
- pkhash: 7
- pkhash: 5
- pkhash: 3
- pkhash: 6
- pkhash: 7
- pkhash: 5
- pkhash: 6
- pkhash: 0
- pkhash: c
- pkhash: 6
- pkhash: 2
- pkhash: 2