Notify users about unavailable linter in the linter widget instead of a 500 error alert

Related to #345156 (closed).

🧩 Problem

When the Linter is down, we show a 500 error at the top of the pipeline editor page.

An alert showing up at the top of the editor can be disruptive to one's workflow and ideally should only be used if we need to draw the user's attention to it so they're aware of an important problem and/or need to take action. A global page alert also usually means a global problem, and here we have a problem specific to the Linter.

Furthermote, if a 500 error occurs with a given config, and the user keep typing, it is very likely that the same error will occur multiple times within 10 seconds. We allow dismissing the error alert banner but it is very likely that after dismissing the alert, in the process of typing if the error happens again and the banner shows up again.

!74543 (merged) fixes the constant scrolling up of page every time the error occurs and alert banner shows up. But in case the user chooses to dismiss the banner, it doesn't show up until a new type of error occurs. There's a chance this can leave the users wondering why the config isn't valid if the alert banner stops showing up.

For more context about the problem see this thread.

✨Proposal

• Remove the 500 error alert when it's caused by the Linter not being available
• Show the Linter status in the linter widget instead (see the mock-up below).
• Show an error alert inside the lint tab when the Linter is not available.

📐 Linter widget status specs

• Configuration validation currently not available. [Learn more]()
• Learn more link should take you to the troubleshooting section in the pipeline editor or Linter docs.
• Use timeout icon in gray-500

📐 Lint tab alert specs

• Use an error alert variant (non-dismissable, it'll show as long as the problem with the Linter persists and the alert should disappear when the Linter becomes available).
• Configuration validation currently not available. [Learn more]()
• Learn more link should take you to the troubleshooting section in the pipeline editor or Linter docs.

Documentation

This issue requires documentation. Learn more link should take you to the troubleshooting section in the pipeline editor or Linter docs where one can learn more about what could cause the linter to be unavailable. It should reassure the user that it doesn't mean that there are problems with configuration.