Description of problem:
Association of importer with invalid importer type should return 404, and not a call report that later will fail.

$ curl -H "Accept: application/json" -X POST  -k -u admin:admin -d '{"importer_type_id": "yum_importer1", "importer_config":{}}' 'https://example.com/pulp/api/v2/repositories/m1/importers/' | python -m json.tool
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   231  100   172  100    59    126     43  0:00:01  0:00:01 --:--:--   126
{
    "error": null,
    "result": null,
    "spawned_tasks": [
        {
            "_href": "/pulp/api/v2/tasks/0f3e8cbd-ea84-445a-9a1d-b29f6d11e922/",
            "task_id": "0f3e8cbd-ea84-445a-9a1d-b29f6d11e922"
        }
    ]
}


$ curl -H "Accept: application/json" -X GET  -k -u admin:admin 'https://example.com/pulp/api/v2/tasks/0f3e8cbd-ea84-445a-9a1d-b29f6d11e922/' | python -m json.tool
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  1277  100  1277    0     0   1530      0 --:--:-- --:--:-- --:--:--  1529
{
    "_href": "/pulp/api/v2/tasks/0f3e8cbd-ea84-445a-9a1d-b29f6d11e922/",
    "_id": {
        "$oid": "533d6e1e3965e4939dfae505"
    },
    "_ns": "task_status",
    "error": {
        "code": "PLP0015",
        "data": {
            "properties": "['importer_type_id']",
            "property_names": [
                "importer_type_id"
            ]
        },
        "description": "Invalid properties: ['importer_type_id']",
        "sub_errors": []
    },
    "finish_time": 1396534814,
    "queue": "reserved_resource_worker-0.compute.internal",
    "start_time": 1396534814,
    "state": "error",
    "tags": [
        "pulp:repository:m1",
        "pulp:action:add_importer"
    ],
    "task_id": "0f3e8cbd-ea84-445a-9a1d-b29f6d11e922",
    "traceback": "Traceback (most recent call last):\n  File \"/usr/lib/python2.7/site-packages/celery/app/trace.py\", line 238, in trace_task\n    R = retval = fun(*args, **kwargs)\n  File \"/usr/lib/python2.7/site-packages/pulp/server/async/tasks.py\", line 369, in __call__\n    return super(Task, self).__call__(*args, **kwargs)\n  File \"/usr/lib/python2.7/site-packages/celery/app/trace.py\", line 416, in __protected_call__\n    return self.run(*args, **kwargs)\n  File \"/usr/lib/python2.7/site-packages/pulp/server/managers/repo/importer.py\", line 117, in set_importer\n    raise InvalidValue(['importer_type_id'])\nInvalidValue: Invalid properties: ['importer_type_id']\n"
}


Version-Release number of selected component (if applicable):
pulp-server-2.4.0-0.3.alpha.git.13.3215df6.fc20.noarch

How reproducible:
always

Steps to Reproduce:
1. create repo
2. associate importer with invalid importer type
3.

Actual results:
Association of importer with invalid importer type returns 202(a task that fails)

Expected results:
Association of importer with invalid importer type returns 404

Additional info: