Remove "set()" from CoreAPIMissing exception

When some core v3 API extensions are not loaded, a CoreAPIMissing
exception happens like the following example:
  CoreAPIMissing: Core API extensions are missing: set(['keypairs'])

"set()" is used only for comparing the loaded extensions and the core
extensions. It is better to remove it from the exception message.

Change-Id: Iddfa69931064a6adb6e3aa7d6efaeb28260be5e0
Merge "Remove "set()" from CoreAPIMissing exception"