500 error when using the submit job ui when requesting a device_type that doesn't exist

[Matthew Hart]

I was trying to use the submit job ui on v.l.o with:

{
    "actions": [
        {
            "command": "deploy_linaro_kernel",
            "parameters": {
                "dtb": "http://community.validation.linaro.org/images/keystone/uImage-rt-k2hk-evm.dtb",
                "firmware": "http://community.validation.linaro.org/images/keystone/skern-keystone-evm.bin",
                "kernel": "http://community.validation.linaro.org/images/keystone/uImage-rt-keystone-evm.bin",
                "ramdisk": "http://community.validation.linaro.org/images/keystone/test.cpio.gz"
            }
        },
        {
            "command": "boot_linaro_image"
        }
    ],
    "device_type": "keystone",
    "health_check": false,
    "job_name": "Keystone Boot Test",
    "logging_level": "DEBUG",
    "timeout": 18000
}

and was getting a HTTP 500 error.
In the uswgi logs I see:

ERROR:django.request:Internal Server Error: /scheduler/jobsubmit
Traceback (most recent call last):
  File "/srv/lava/.cache/eggs/Django-1.4.2-py2.7.egg/django/core/handlers/base.py", line 111, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "/srv/lava/.cache/git-cache/exports/lava-scheduler/2013-09-18-a38e6cf/lava_scheduler_app/views.py", line 600, in job_submit
    request.POST.get("json-input"), request.user)
  File "/srv/lava/.cache/git-cache/exports/lava-scheduler/2013-09-18-a38e6cf/lava_scheduler_app/models.py", line 481, in from_json_and_user
    device_type = DeviceType.objects.get(name=job_data['device_type'])
  File "/srv/lava/.cache/eggs/Django-1.4.2-py2.7.egg/django/db/models/manager.py", line 131, in get
    return self.get_query_set().get(*args, **kwargs)
  File "/srv/lava/.cache/eggs/Django-1.4.2-py2.7.egg/django/db/models/query.py", line 366, in get
    % self.model._meta.object_name)
DoesNotExist: DeviceType matching query does not exist.

Which showed that I was trying to request a device type that didn't exist.

It would be much better if the UI was able to give me this error rather than just the HTTP 500 page.