activation: extension crashes if go is not found

[hyangah]

From #457 (comment)

The extension host crashed with the following trace.

[2020-09-18 15:50:29.097] [exthost] [error] Activating extension golang.go failed due to an error:
[2020-09-18 15:50:29.103] [exthost] [error] TypeError: Cannot read property 'then' of undefined
	at activate (/Users/aran/.vscode/extensions/golang.go-0.16.2/dist/goMain.js:13951:46)
	at Function._callActivateOptional (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/services/extensions/node/extensionHostProcess.js:917:841)
	at Function._callActivate (/Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/services/extensions/node/extensionHostProcess.js:917:492)
	at /Applications/Visual Studio Code.app/Contents/Resources/app/out/vs/workbench/services/extensions/node/extensionHostProcess.js:915:724
	at processTicksAndRejections (internal/process/task_queues.js:94:5)
	at async Promise.all (index 0)

The crash point corresponds to

vscode-go/src/goMain.ts

Line 85 in 5c4b766

updateGoVarsFromConfig().then(async () => {

I suspect, the user's vscode got launched with incomplete PATH after the reboot & clean restart,
so the extension failed to find the go. The extension is supposed to a notification popup to indicate
this issue, but it's not guaranteed that the popup would show up first before the extension crashes.

[gopherbot]

Change https://golang.org/cl/255963 mentions this issue: src/goInstallTools.ts: return a rejected promise when go isn't found

[gopherbot]

Change https://golang.org/cl/257603 mentions this issue: [release] src/goInstallTools.ts: return a rejected promise when go isn't found