Provide more information on unavailable web app connection

[stevenGravy]

Expected behavior:

Provide more information to the user that a connection failed and how to resolve.

Current behavior:

If the app access service can't reach a web url the user just gets Bad Gateway. The user wouldn't be clear this is Teleport failing to connect or their own browser.

Bug details:

• Teleport version: 16.4.2
• Recreation steps

1. Add a app service with a url that isn't connected.

  - name: "grafana"
    uri: "http://localhost:3000"
    public_addr: "grafana.enterprise.teleportdemo.com"
    labels:
      env: dev

2. Attempt to access from the web ui