Improve error message of fetch when xhr.onerror occurs (#3003)

* Ensure fetch-tests fail (instead of timeout) on failure

* Standardize failure handling in fetch-tests

* Improve flawed failure condition of test_fetch_json

Previously, the assertion was always true

* Implement better error message for for xhr.onerror within fetch

tns-core-modules/fetch/fetch.js

@@ -310,8 +310,8 @@
                 resolve(new Response(xhr.responseText, options))
             }
 
-            xhr.onerror = function () {
-                reject(new TypeError('Network request failed'))
+            xhr.onerror = function (error) {
+                reject(new TypeError(['Network request failed:', error.message].join(' ')))
             }
 
             xhr.open(request.method, request.url, true)