improve GOTOOLCHAIN switch experience

[hyangah]

Inspired by golang/go#63922

When the target toolchain version can't be downloaded, currently, the VS Code Go extension fails hard. It will be nice if the extension fails gracefully and guides the user to successfully address the issue.

Issues

• Better logging - the extension runs go version and go env commands during activation. The error messages logged in the Go output channels are the result of these command runs, but revealed out of context. That results in confusion. Log why extension runs the command, and what exact command it runs. And, if possible, how to fix them.
• Missing Go status bar - the activation failed before registering the status bar, so the go status bar didn't appear. Instead, the extension should continue registering the status bar and make the problem visible (changing color or showing error icon).
• Incomplete command registration - since activation couldn't complete, most commands weren't registered. However, they appear in the Command Palette. VS Code allows conditional activation of commands and ui components. Utilize it. And some commands (e.g. restart language server or reactive) may be useful when user fixes the issue and recover.
• Gopls not started - I think we can continue to start gopls so helpful error messages are coming from gopls.
• Update the Go status bar version when the toolchain version in use is changed.

cc @golang/tools-team

[gopherbot]

Change https://go.dev/cl/550918 mentions this issue: src/goExplorer: add GOTOOLCHAIN in the go env var explorer

[firelizzard18]

I get similar failures to #3215 if I set "go.toolsEnvVars": { "GOEXPERIMENT": "rangefunc" }, with the Go version configured to 1.21.

[chriscasola]

We're using a vscode dev container (mcr.microsoft.com/vscode/devcontainers/go:1.22-bullseye) and it or the go extension seem to be setting GOTOOLCHAIN=local, which disables automatic toolchain selection. I'm not sure the reasoning for this, but it would be nice if that did not happen, or if there was a way to use GOTOOLCHAIN=auto instead.