Improve message description for limit errors (a.k.a 429 Too many Request)

[dgaubert]

We need to improve this error message, to make sure that the user understands either his query timed out or he reached his rate-limit.

{
  error: [
    "You are over platform's limits. Please contact us to know more details"
  ],
  context: "limit",
  detail: "datasource"
}