Fix page load for proxied containers when Mozilla VPN isn't active (fix #2570)

[lusimeon]

Before submitting your pull request

• I agree to license my code under the MPL 2.0 license.
• I rebased my work on top of the main branch.
• I ran npm test and all tests passed.
• I added test coverages if relevant.

Description

Remove the use of the proxy if Mozilla VPN inactive, still load page in the dedicated container. Without this fix, infinite page load happen on custom location proxied containers if Mozilla VPN is inactive (more info on #2570).

To reproduce:

• Assign a custom VPN location to a container.
• Globally disable Mozilla VPN.
• Open a page with this container.
• The page will load indefinitely.

Type of change

• Bug fix
• New feature
• Major change (fix or feature that would cause existing functionality to work differently than in the current version)
  • The container will not use proxy even if it is configured to work behind a proxy when Mozilla VPN is inactive (globally disabled or not installed).

Tag issues related to this pull request:

• Unable to use VPN only for selected tabs #2570

[3nprob]

The proposed solution might be dangerous? Could cause involuntary VPN leak.

A specific error page explaining the situation and why the request was blocked (perhaps with a button to pass through anyway - similar to the flow for invalid TLS certs?) seems more on-point (albeit a more involved implementation)?