Detect when Tor is blocked on the user's network and warn about it

[fmarier]

Some users report that their Tor windows don't work and that the Tor daemon is stuck in the Disconnect state without any log messages:

• https://community.brave.com/t/private-tabs-with-tor-is-always-disconnected/248390/13
• https://community.brave.com/t/tor-is-always-disconnected/473164/29

The root cause in this case was the use of OpenDNS with the "Anonymizer/Proxy" blocking enabled.

I can think of three ways to tackle this:

1. Warn the user when the tor daemon cannot be downloaded for any reason (generic error message).
2. Make a request (not on the Tor network) to https://tor.bravesoftware.com/ and look for a 200. This requires a small server-side fix first: https://github.com/brave/devops/issues/9329.
3. Make a request (not on the Tor network) https://www.torproject.org and look for a 200.

I think we should do both #1 and #2 with these errors messages respectively:

1. "Could not download the required tor component."
2. "Requests for the tor component appear to be blocked on this network."

[rmcfadden3]

Per Slack convo w/ @fmarier — the error message copy LGTM!

[fmarier]

After talking to @boocmp , it turns out that we can't easily tell between these two conditions:

1. The component is missing from disk because the network blocks it.
2. The component is missing from disk because it hasn't finished downloading.

He suggested showing a "downloading..." message instead.

So maybe what we should do instead is:

1. Start by checking for a 200 on tor.bravesoftware.com and return an error message immediately if it fails.
2. If step 1 succeeded, start downloading with the "Downloading tor daemon..." status message.