switch to fetch for checking for the test file

getLocalFile intentionally doesn't cope with errors, so it was the wrong choice.

This could have been causing real errors on the options page (as `loaded` wasn't being set to true), but I didn't observe any problems.

src/common/options.js

@@ -113,14 +113,25 @@ function onLoad() {
   // Hide the tests link if the page isn't available. It may be stripped out
   // of extension packages.
 
-  // Check if our test file exists.
-  Utils.getLocalFile('./test/index.html', 'text', function(_, err) {
-    // The test files aren't present, so hide the button.
-    if (err) {
+  // Check if our test file exists. Note that we can't use Utils.getLocalFile as it throws
+  // an asynchronous error if the file isn't found.
+  // TODO: When Utils.getLocalFile is changed to return a promise, use it here.
+  fetch('./test/index.html')
+    .then(response => {
+      if (!response.ok) {
+        // The test files aren't present, so hide the button.
+        $('#tests-link').hide();
+      }
+      else {
+        // When the file is absent, Firefox still gives a 200 status, but will throw an
+        // error when the response is read.
+        return response.text();
+      }
+    })
+    .catch(err => {
       // The test files aren't present, so hide the button.
       $('#tests-link').hide();
-    }
-  });
+    });
 
   // Older Thunderbird may try to open this options page in a new ChromeWindow, and it
   // won't work. So in that case we need to tell the user how they can actually open the

src/common/utils.js

@@ -339,7 +339,10 @@ function getLocalURL(url) {
 // Makes an asynchrous XHR request for a local file (basically a thin wrapper).
 // `dataType` must be one of 'text', 'json', or 'base64'.
 // `callback` will be called with the response value, of a type depending on `dataType`.
-// Errors are no expected for local files, and will result in an exception being thrown asynchrously.
+// Errors are not expected for local files, and will result in an exception being thrown asynchrously.
+// TODO: Return a promise instead of using a callback. This will allow returning an error
+// properly, and then this can be used in options.js when checking for the existence of
+// the test file.
 function getLocalFile(url, dataType, callback) {
   fetch(url)
     .then(response => {

src/manifest.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "__MSG_app_name__",
-  "version": "2.14.0",
+  "version": "2.14.1",
   "description": "__MSG_app_slogan__",
   "homepage_url": "https://markdown-here.com",
   "default_locale": "en",