Files App: Minor code refactoring

No functional change.

BUG=chromium:912236
TEST=Manual

Change-Id: Ie7a79682726701f2aad255183e0b2d674cf3e26b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462991Reviewed-by: Jeremie Boulic <jboulic@chromium.org>
Reviewed-by: François Degros <fdegros@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817722}

ui/file_manager/file_manager/foreground/js/file_tasks.js

@@ -871,7 +871,7 @@ class FileTasks {
     const filename = util.extractFilePath(url).split('/').pop();
 
     const item = new ProgressCenterItem();
-    item.id = 'mount-' + url;
+    item.id = 'Mounting: ' + url;
     item.type = ProgressItemType.MOUNT_ARCHIVE;
     item.message = strf('ARCHIVE_MOUNT_MESSAGE', filename);
 
@@ -962,14 +962,13 @@ class FileTasks {
 
       const filename = util.extractFilePath(url).split('/').pop();
       const item = new ProgressCenterItem();
-      item.id = 'cannot-mount-' + url;
+      item.id = 'Cannot mount: ' + url;
       item.type = ProgressItemType.MOUNT_ARCHIVE;
       item.message = strf('ARCHIVE_MOUNT_FAILED', filename);
       item.state = ProgressItemState.ERROR;
       this.progressCenter_.updateItem(item);
 
-      console.error(`Cannot mount '${url}':`);
-      console.error(error);
+      console.error(`Cannot mount '${url}': ${error.stack || error}`);
     }
   }