Cloud license activation error: Surface silent mode message in UI

Problem to solve

Our docs provide guidance on troubleshooting the An error occurred while adding your subscription error message when activating a cloud subscription on the instance.

Having silent mode enabled is a possible cause: https://docs.gitlab.com/ee/administration/license.html#error-an-error-occurred-while-adding-your-subscription

The user needs to use the browser's Developer Tools to find additional information on the error, since the message is generic.

Proposal

If it can be determined that silent mode is the cause of the error, we should adjust the error message (An error occurred while adding your subscription) to be more specific. A specific error message saves the user the additional troubleshooting steps and prevents Support tickets.

Current	Proposed
An error occurred while adding your subscription	Subscription cannot be activated when [silent mode](https://docs.gitlab.com/ee/administration/silent_mode) is enabled. Disable silent mode and try again.