fix(frontend): improve network error handling while waiting for results

Previously a network error stopped the result page from polling and the user only saw an endless spinner.
Now polling goes on even if flask is temporarily not available, e.g. while restarting.
Polling goes on until flask sends a status of SUCCESS or ERROR

Closes: #280

client-src/shared/poll/poll.js

@@ -67,11 +67,11 @@ async function poll(url, prefix) {
     }
 
     try {
-        return PollUntilValid.poll(url, validateFn, 1000, onValid, onProgress, onError);
+        return await PollUntilValid.poll(url, validateFn, 1000, onValid, onProgress, onError);
     } catch (e) {
         // Network Error or other reason why the request could not be completed
         console.log(e);
-        handleError(prefix);
+        handleError(prefix, e);
         return null;
     }
 }

client-src/shared/poll/pollUntilValid.js

@@ -27,8 +27,24 @@ class PollUntilValid {
         onError = () => {},
     ) {
         do {
-            /* eslint-disable no-await-in-loop */
-            const response = await fetch(url);
+            let response;
+            try {
+                /* eslint-disable no-await-in-loop */
+                response = await fetch(url);
+            } catch (error) {
+                if (error instanceof TypeError) {
+                    // Chrome and Firefox use different Error messages, so it's hard to be more
+                    // specific than checking for TypeError
+                    // see: https://developer.mozilla.org/en-US/docs/Web/API/fetch#exceptions
+                    console.log("NetworkError, continue retrying", error);
+                    await PollUntilValid.wait(5000);
+                } else {
+                    throw error;
+                }
+            }
+
+            // eslint-disable-next-line no-continue
+            if (!response) continue;
 
             // if response code is not between 200-299
             if (!response.ok) {
@@ -43,6 +59,7 @@ class PollUntilValid {
                 await onValid(response);
                 return response;
             }
+
             /* eslint-enable */
         } while (true);
     }