Display a readable message when the Eno API returns an error

[romaintailhurat]

Currently, when we get an error response from the Eno API, we apply the same behaviour as a successfull one: a browser tab is open, displaying a 500 error message, not very understandable by the end user.

We propose the following change: when it's an error status, the error message is displayed as a information modal, with an understandable cause.

In order to have something the user can understant, two options:

1. the Eno API provides us with this proper message
2. we translate a maybe too technical message in something clearer