[Download][TDAF-1353] Exception fix for 'start' function

[Download][TDAF-1353] Exception fix for 'start' function

`start` function was returning unknown error exception instead of
unsupported error in case when networkType in DownloadRequest wasn't
supported by the device.

Additionally fix enables null values to be used (in accordance with the documentation).

[Verification] Code builds without errors. TCT suites deprecated, download and systeminfo pass rate: 100%.
Tested in developer console on devices with telephony set to true and false.
Below code was used with all network types, as well as invalid values.
```
var downloadRequest = new tizen.DownloadRequest(
                    "http://download.tizen.org/tct/2_1/webapi-tizen-download-test-image-lq.png",
                    null, null, "CELLULAR", null);

tizen.download.start(downloadRequest);
```

Change-Id: I4f2866a07019f129c852024970783b110ef11abc
Signed-off-by: Arkadiusz Pietraszek <a.pietraszek@samsung.com>