fix: handle new 403 "redirect" responses from backend

To make it easier to redirect the user to an another page or an
access denied error page, this PR deprecates the old custom 401 &
404 error handling, replacing them both with a generic 403 error
code that contains a URL to redirect the user to.

Before this change, we weren't handling all the error cases, and
when we didn't, we ended up showing a broken page to the user.

src/course-home/data/api.js

@@ -182,6 +182,26 @@ export function normalizeOutlineBlocks(courseId, blocks) {
   return models;
 }
 
+function processTabDataErrorRedirect(error) {
+  const { httpErrorResponseData, httpErrorStatus } = error && error.customAttributes;
+
+  if (httpErrorStatus === 403) {
+    // Currently, only 403 errors contain redirect content
+    try {
+      const { redirect } = JSON.parse(httpErrorResponseData);
+      if (redirect) {
+        global.location.replace(redirect);
+        return true;
+      }
+    } catch (exc) {
+      // ignore any json parse errors, might be an actual 403 without redirect json content
+    }
+  }
+
+  // Did not do any redirecting
+  return false;
+}
+
 export async function getCourseHomeCourseMetadata(courseId) {
   let url = `${getConfig().LMS_BASE_URL}/api/course_home/v1/course_metadata/${courseId}`;
   url = appendBrowserTimezoneToUrl(url);
@@ -202,11 +222,18 @@ export async function getDatesTabData(courseId) {
   } catch (error) {
     const { httpErrorStatus } = error && error.customAttributes;
     if (httpErrorStatus === 404) {
+      // TODO: remove this - not needed once the backend uses 403s for redirects
       global.location.replace(`${getConfig().LMS_BASE_URL}/courses/${courseId}/dates`);
+      return null;
     }
     // 401 can be returned for unauthenticated users or users who are not enrolled
     if (httpErrorStatus === 401) {
+      // TODO: remove this - not needed once the backend uses 403s for redirects
       global.location.replace(`${getConfig().BASE_URL}/course/${courseId}/home`);
+      return null;
+    }
+    if (processTabDataErrorRedirect(error)) {
+      return null; // keeps loading screen active
     }
     throw error;
   }
@@ -249,11 +276,18 @@ export async function getProgressTabData(courseId, targetUserId) {
   } catch (error) {
     const { httpErrorStatus } = error && error.customAttributes;
     if (httpErrorStatus === 404) {
+      // TODO: remove this - not needed once the backend uses 403s for redirects
       global.location.replace(`${getConfig().LMS_BASE_URL}/courses/${courseId}/progress`);
+      return null;
     }
     // 401 can be returned for unauthenticated users or users who are not enrolled
     if (httpErrorStatus === 401) {
+      // TODO: remove this - not needed once the backend uses 403s for redirects
       global.location.replace(`${getConfig().BASE_URL}/course/${courseId}/home`);
+      return null;
+    }
+    if (processTabDataErrorRedirect(error)) {
+      return null; // keeps loading screen active
     }
     throw error;
   }
@@ -303,8 +337,12 @@ export async function getOutlineTabData(courseId) {
   } catch (error) {
     const { httpErrorStatus } = error && error.customAttributes;
     if (httpErrorStatus === 404) {
+      // TODO: remove this - not needed once the backend uses 403s for redirects
       global.location.replace(`${getConfig().LMS_BASE_URL}/courses/${courseId}/course/`);
-      return {};
+      return null;
+    }
+    if (processTabDataErrorRedirect(error)) {
+      return null; // keeps loading screen active
     }
     throw error;
   }

src/course-home/data/thunks.js

@@ -49,6 +49,11 @@ export function fetchTab(courseId, tab, getTabData, targetUserId) {
         logError(courseHomeCourseMetadataResult.reason);
       }
 
+      if (fetchedTabData && tabDataResult.value === null) {
+        // null tab data indicates that we have redirected elsewhere - just exit and don't visibly stop loading
+        return;
+      }
+
       if (fetchedTabData) {
         dispatch(addModel({
           modelType: tab,