Upgrade some logging to possibly detect the screensaver not starting.

There are reports of the screensaver not starting. The only way
I can see this happening is if there are no topics returned by IMAX
and the cache is empty (which can happen if you just changed screensaver
settings in the hub since we clear the cache).

I've upgraded some logs that were previously at DVLOG that I think are
notable errors, possibly explaining why IMAX would give us no topics.

Bug: b:228221864
Bug: b:228221764
Change-Id: If899c9f1112099dfcbede3d73f48ab499e411314
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3579734
Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Eric Sum <esum@google.com>
Cr-Commit-Position: refs/heads/main@{#990684}

ash/ambient/backdrop/ambient_backend_controller_impl.cc

@@ -328,12 +328,11 @@ class BackdropURLLoader {
       response_code = simple_loader_->ResponseInfo()->headers->response_code();
     }
 
-    DVLOG(2) << "Downloading Backdrop proto failed with error code: "
-             << response_code << " with network error"
-             << simple_loader_->NetError();
+    LOG(ERROR) << "Downloading Backdrop proto failed with error code: "
+               << response_code << " with network error"
+               << simple_loader_->NetError();
     simple_loader_.reset();
     std::move(callback).Run(std::make_unique<std::string>());
-    return;
   }
 
   std::unique_ptr<network::SimpleURLLoader> simple_loader_;
@@ -449,7 +448,7 @@ void AmbientBackendControllerImpl::FetchScreenUpdateInfoInternal(
     const std::string& gaia_id,
     const std::string& access_token) {
   if (gaia_id.empty() || access_token.empty()) {
-    DVLOG(2) << "Failed to fetch access token";
+    LOG(ERROR) << "Failed to fetch access token for ScreenUpdate";
     // Returns an empty instance to indicate the failure.
     std::move(callback).Run(ash::ScreenUpdate());
     return;