DEV: Improve external upload debugging (#28627)

* Do not delete created external upload stubs for 2 days
  instead of 1 hour if enable_upload_debug_mode is true,
  this aids with server-side debugging.
* If using an API call, return the detailed error message
  if enable_upload_debug_mode is true. In this case the user
  is not using the UI, so a more detailed message is appropriate.
* Add a prefix to log messages in ExternalUploadHelpers, to
  make it easier to find these in logster.

app/models/external_upload_stub.rb

@@ -21,7 +21,7 @@ class ExternalUploadStub < ActiveRecord::Base
           where(
             "status = ? AND created_at <= ?",
             ExternalUploadStub.statuses[:created],
-            CREATED_EXPIRY_HOURS.hours.ago,
+            (SiteSetting.enable_upload_debug_mode ? 48 : CREATED_EXPIRY_HOURS).hours.ago,
           )
         end