Update errors for Subscription Activation Alert

What does this MR do?

Relates to gitlab-org/gitlab#331148

With this MR we:

• update the copy and links for the general error during subscription activation
• add an error for when the activation code is invalid

A feature flag is not used because this feature is for self-managed instances. The feature is behind an Application Setting and it's not released nor documented. To test it locally, see the following steps.

Local Testing

First subscription activation

1. Delete any previous license you might have

2. Make sure you have set the following environment vars (usually in your terminal) and restart gdk

   • GITLAB_LICENSE_MODE is set to test
   • CUSTOMER_PORTAL_URL is set to https://customers.stg.gitlab.com

3. Use Rails console to run ApplicationSetting.current.update(cloud_license_enabled: true)

4. Visit Admin > Subscription

   A. Enter whatever (even literally) as an activation code and see the error appearing > See the activation code error B. Block the request localhost:3000/api/graphql > See the general error

Connectivity Issue

1. Set CUSTOMER_PORTAL_URL to a-fake-address and run gdk restart, enter

2. Visit Admin > Subscription

   A. Enter whatever (even literally) as an activation code and see the error appearing > See the connectivity Issue error

Screenshots (strongly suggested)

General Error	Code Activation Error	Connectivity Issue

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #331148