[ASCII-1924] Improve error printing and help messages in the Agent GUI

[misteriaud]

What does this PR do?

This PR enhances the error messages displayed to the customer for scenarios such as invalid token, session expired, or Agent unreachable.

Here is an example of the page displayed to the user when logged out:

Motivation

This PR aims to provide better information in case of errors when using the GUI.

Describe how to test/QA your changes

• (run the Agent)
• run the command datadog-agent launch-gui
• check that everything is working as before
• add a session expiration with the following parameter in datadog.yaml : GUI_session_expiration: 20s
• restart the Agent
• run the command datadog-agent launch-gui
• check that after ~20s you get the error displayed

[pr-commenter[bot]]

Test changes on VM

Use this command from test-infra-definitions to manually test this PR changes on a VM:

inv create-vm --pipeline-id=38485419 --os-family=ubuntu

Note: This applies to commit f84ed81d

[misteriaud]

/merge

[dd-devflow[bot]]

:steam_locomotive: MergeQueue: pull request added to the queue

The median merge time in main is 26m.

Use /merge -c to cancel this operation!