title: Ajv custom error message for type
body: <p>i was exploring Ajv with ajv-errors for validating json schema and producing custom error messages. everything works as of now but i can't set custom error message for type for individual values.</p>
  
  <pre><code>const emailSchema = {
   type: 'object',
   required: ['foo', 'bar', 'car'],
   properties: {
    foo: { type: 'integer' },
    bar: { type: 'string' },
    car: { type: 'string' }
   },
   errorMessage: {
    type: 'should be an object',
    required: {
    foo: 'foo field is missing',
    bar: 'bar field is missing',
    car: 'car field is missing'
    }
   } 
  };
  </code></pre>
  
  <p>outputs following error </p>
  
  <pre><code>[
      {
          "keyword": "type",
          "dataPath": "/foo",
          "schemaPath": "#/properties/foo/type",
          "params": {
              "type": "integer"
          },
          "message": "should be integer"
      },
      {
          "keyword": "errorMessage",
          "dataPath": "",
          "schemaPath": "#/errorMessage",
          "params": {
              "errors": [
                  {
                      "keyword": "required",
                      "dataPath": "",
                      "schemaPath": "#/required",
                      "params": {
                          "missingProperty": "bar"
                      },
                      "message": "should have required property 'bar'"
                  }
              ]
          },
          "message": "bar field is missing"
      },
      {
          "keyword": "errorMessage",
          "dataPath": "",
          "schemaPath": "#/errorMessage",
          "params": {
              "errors": [
                  {
                      "keyword": "required",
                      "dataPath": "",
                      "schemaPath": "#/required",
                      "params": {
                          "missingProperty": "car"
                      },
                      "message": "should have required property 'car'"
                  }
              ]
          },
          "message": "car field is missing"
      }
  ]
  </code></pre>
  
  <p>the first error object with message "should be integer", can i customize it like foo must be an Integer.
  I am expecting something like below but it gives be schema error.</p>
  
  <pre><code>type : {
    foo : "foo must be an Integer"
  }
  </code></pre>
  
  <p>Thanks.</p>