Handle custom field errors and allow them to be added to creation payload

[Skiggz]

This will be pretty involved, but an example trying to create an epic:

{"errorMessages":[],"errors":{"customfield_123":"Epic Name is required."}}

payload

{
  "fields": {
    "summary": "An epic, epic, yo",
    "project": {
      "id": "123"
    },
    "issuetype": {
      "id": "222"
    },
    "priority": {
      "id": "3"
    },
    "description": "Some description"
  }
}