Improve fetch() error messages

src/safer-fetch.js

@@ -11,7 +11,6 @@ const queue = [];
 const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
 
 const startNextFetch = ([resolve, url, options]) => {
-    let firstError;
     let failedAttempts = 0;
 
     /**
@@ -43,17 +42,16 @@ const startNextFetch = ([resolve, url, options]) => {
             // if we send too many at once (as we do).
 
             console.warn(`Attempt to fetch ${url} failed`, error);
-            if (!firstError) {
-                firstError = error;
-            }
 
             if (failedAttempts < 2) {
                 failedAttempts++;
                 sleep((failedAttempts + Math.random() - 1) * 5000).then(attemptToFetch);
                 return;
             }
 
-            done(Promise.reject(firstError));
+            // The fetch() error is usually very generic, so we'll add enough information
+            // to possibly be useful.
+            done(Promise.reject(new Error(`Storage request ${url} failed: ${error}`)));
         });
 
     attemptToFetch();

test/unit/tw-get-error.test.js

@@ -0,0 +1,30 @@
+jest.mock('cross-fetch', () => {
+    const crossFetch = jest.requireActual('cross-fetch');
+    const mockFetch = () => Promise.reject(new Error('Intentional error for testing'));
+    return {
+        ...crossFetch,
+        default: mockFetch,
+        fetch: mockFetch
+    };
+});
+
+const FetchTool = require('../../src/FetchTool');
+
+jest.setTimeout(15000);
+
+test('get() returns a somewhat useful error message', async () => {
+    const tool = new FetchTool();
+
+    let error = null;
+    try {
+        await tool.get({
+            url: 'https://something.turbowarp.org/1234.png'
+        });
+    } catch (e) {
+        error = e;
+    }
+
+    expect(error.toString()).toMatch(
+        /Storage request https:\/\/something.turbowarp.org\/1234.png failed: Error: Intentional error for testing/
+    );
+});