Modernize the calculateMD5 test helper function

The test helper code largely predates the introduction of modern
JavaScript features and should be refactored to improve readability.
In particular callbacks and recursive function calls make the code
harder to understand and maintain.

This commit:

- replaces the callback argument with returning a promise;
- uses `const` instead of `var`;
- uses arrow functions for shorter code;
- uses template strings for shorter string formatting code;
- improves the error messages to have more details.

test/downloadutils.mjs

@@ -110,23 +110,18 @@ function downloadManifestFiles(manifest, callback) {
   downloadNext();
 }
 
-function calculateMD5(file, callback) {
-  var hash = crypto.createHash("md5");
-  var stream = fs.createReadStream(file);
-  stream.on("data", function (data) {
-    hash.update(data);
-  });
-  stream.on("error", function (err) {
-    callback(err);
-  });
-  stream.on("end", function () {
-    var result = hash.digest("hex");
-    callback(null, result);
+function calculateMD5(file) {
+  return new Promise((resolve, reject) => {
+    const hash = crypto.createHash("md5");
+    const stream = fs.createReadStream(file);
+    stream.on("data", data => hash.update(data));
+    stream.on("error", error => reject(error));
+    stream.on("end", () => resolve(hash.digest("hex")));
   });
 }
 
 function verifyManifestFiles(manifest, callback) {
-  function verifyNext() {
+  async function verifyNext() {
     if (i >= manifest.length) {
       callback(error);
       return;
@@ -150,35 +145,29 @@ function verifyManifestFiles(manifest, callback) {
       verifyNext();
       return;
     }
-    calculateMD5(item.file, function (err, md5) {
-      if (err) {
-        console.log('WARNING: Unable to open file for reading "' + err + '".');
-        error = true;
-      } else if (!item.md5) {
+
+    try {
+      const md5 = await calculateMD5(item.file);
+      if (!item.md5) {
         console.error(
-          'WARNING: Missing md5 for file "' +
-            item.file +
-            '". ' +
-            'Hash for current file is "' +
-            md5 +
-            '"'
+          `WARNING: MD5 hash missing for "${item.file}" (computed "${md5}").`
         );
         error = true;
       } else if (md5 !== item.md5) {
         console.error(
-          'WARNING: MD5 of file "' +
-            item.file +
-            '" does not match file. Expected "' +
-            item.md5 +
-            '" computed "' +
-            md5 +
-            '"'
+          `WARNING: MD5 hash mismatch for "${item.file}" (expected "${item.md5}", computed "${md5}").`
         );
         error = true;
       }
-      i++;
-      verifyNext();
-    });
+    } catch (ex) {
+      console.log(
+        `WARNING: MD5 hash calculation failed for "${item.file}" ("${ex}").`
+      );
+      error = true;
+    }
+
+    i++;
+    verifyNext();
   }
   var i = 0;
   var error = false;