ENH Work with official list of supported modules

[emteknetnz]

Issue silverstripe/cwp#320

[GuySartorelli]

Update the PHPDoc for the ApiLoader class - it says "Handles fetching supported addon details from addons.silverstripe.org" but that's not where we're fetching the data from.
Also update phpdoc for doRequest() - this method doesn't seem to have anything to do with checking "module health"?

Also given that we're touching the error messaging here, we should update the "response could not be parsed" error

if (empty($responseJson)) {
-    throw new RuntimeException($failureMessage . 'Response could not be parsed');
+    if (json_last_error() !== JSON_ERROR_NONE) {
+        throw new RuntimeException($failureMessage . json_last_error_msg());
+    }
+    throw new RuntimeException($failureMessage . 'Response was empty');
}

Make sure to add testing for both of these failure scenarios.