Send kubectl connection errors to frontend

What does this MR do?

The current Cluster Index View has a feature flag clusters_list_redesign which is being built to display nodes and memory on the cluster table

These nodes are being queried from an external Kubernetes cluster. The query can fail for various reasons, resulting in the table displaying unknown in the node columns

We have 2 follow-up issues

1. Send errors messages to the Frontend: #222951 (closed)
2. Use those error messages to display failure information to the user: #220182 (closed)

This MR will address issue 1. The MR will:

• Add a new field connection_error (This was changed to 3 error messages during the MR review) to the cluster serializer, which will send error statuses to the Frontend.
• Attempt to capture error messages in cluster model and the node module
• Update associated tests

Screenshots

ConnectionError

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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 #222951 (closed)