Feat: format API validation errors

[jonathandavies-CS]

This corrects logic to format API errors like so:

{
  "errors": {
    "foo": [
      "Foo is required"
    ],
    "email": [
      "'bad-email' is not a valid email address"
    ],
    "bar": [
      "Bar is not out of bounds, 1000 is greater than 255"
    ]
  }
}

Note this only works when a struct has been extended and given validation tags, there exists a PR to further implement errr formatting for OSCAL components:

#286