title: Partially strict marshmallow schema
body: <p>I have a Flask Rest Api which uses marshmallow for serialization-desirealization. The Api recieves a batch of items.</p>
  
  <p><strong>I'd like to perform a strict validation on the input schema per item instead of on the entire request.</strong> 
  The purpose of this is to allow me to do fine-grained error handling, i.e. process the entire batch request and also return validation errors for the bad Items.</p>
  
  <p>Example Time!</p>
  
  <p>Suppose this is my main Rest Json Request Schema (note the list of <code>Nested</code> fields) - </p>
  
  <pre><code>from marshmallow import Schema
  
  class MyRequestSchema(Schema):
      class Meta:
          strict = True
      items = fields.List(fields.Nested(MyItemSchema), required=True)
      id = fields.String(required=False, missing=None)
      meta_field = fields.Integer(required=False, missing=False)
  </code></pre>
  
  <p>And this is my <code>Nested field</code> schema- </p>
  
  <pre><code>class MyItemSchema(Schema):
      class Meta:
          strict = True
      name = fields.String(required=False, missing=None)
      flag = fields.Bool(required=False, missing=False)
  </code></pre>
  
  <p>what I'd like is for marshmallow to 1) deserialize the entire request schema and 2) Supply me with per-item schema validation errors or a serialized object.
  Using this behavior, I would process each <code>Item</code>. For every Item which contains validation errors, I'll return an error object and for the valid Items I'll return a proper response Item.</p>
  
  <p>So for a request like this - </p>
  
  <pre><code>{
    "id": "d6f6f80c558a11",
    "meta_field": 42,
    "items": [
      {
        "name": "abc",
        "flag": true
      },
      {
        "bad_field": "bad_value"
      }
    ]
  }
  </code></pre>
  
  <p>I'd like to return a response along the lines of this - </p>
  
  <pre><code>{
    "id": "d6f6f80c558a11",
    "result_items": [
      {
        "name": "abc",
        "result": "Great Success!"
      },
      {
        "schema_validation_error": {
          "details": {
            "listings": {
              "0": {
                "name": [
                  "Missing data for required field."
                ],
                "flag": [
                  "Missing data for required field."
                ]
              }
            }
          },
          "message": "Invalid Item"
        }
      }
    ]
  }
  </code></pre>
  
  <p>Does marshmallow support this behaviour?</p>
  
  <p>Any suggestions on implementing this?</p>
  
  <p>Ideally, I'd like an answer which embraces as much of Flask+Marshmallow as possible (instead of implementing the logic myself)</p>
  
  <p>Thanks!</p>