Process graph validation returns only a single error

[m-mohr]

Currently, the validation of process graphs (POST /validation) can only return a single error and a user would need to request a list of errors for individual error. It would be much more user-friendly if the user could get a list of all errors in a single request.

List of errors could be returned as follows:

{
  "errors":[
    {
      "id":"1541025804936",
      "code":2103,
      "message":"The value specified for the process argument 'extent' in process 'filter_bbox' is invalid: instance.north is not of a type(s) number",
      "url":"https://open-eo.github.io/openeo-api/errors/#openeo-error-codes"
    },
    {
      "id":"1541026583276",
      "code":2103,
      "message":"The value specified for the process argument 'extent' in process 'filter_bbox' is invalid: instance additionalProperty \"base\" exists in instance when not allowed",
      "url":"https://open-eo.github.io/openeo-api/errors/#openeo-error-codes"
    }
  ],
  "links":[
  ]
}

[m-mohr]

Process graph validation (POST /validation) now returns a 200 HTTP status code for both valid and invalid process graphs and responds with a list of errors. Validation errors are still a valid response for a validation request and thus should respond with a success status code such as 200.