Extension does not properly start when no CodeQL CLI is found

拡張機能の起動フロー、CodeQL CLI の実行可能ファイルパスの処理、および再現手順で説明されている更新チェックの経路から始めます。無効な CLI パスの場合とダウンロードフォルダーを削除した場合に再現し、その後ログと「CodeQL: Quick query」コマンドの動作を確認します。完了の条件は、起動時に CLI が見つからないことを報告し、設定されたパスを尊重し、設定を変更しなくてもインストール操作が機能することです。

Describe the bug
It looks like when no CodeQL CLI is found on start up the extension does not start properly. This is noticable as:

• No "CodeQL" / "CodeQL CLI v ..." is shown in the bottom right corner
• CodeQL CLI is not downloaded automatically? See #900
• Once an extension config change is made, the extension seems to start properly, downloading CodeQL CLI; even if the config change was specifying a custom CodeQL CLI executable path

Edit: Might only occur when update check was performed recently and the following was logged:

Didn't perform CodeQL CLI update check since a check was already performed within the previous 86400 seconds.

Version
Extension:

CodeQL extension version: 1.5.1 
CodeQL CLI version: 2.5.7 
Platform: win32 x64

VSCode:

Version: 1.57.1 (user setup)
Commit: 507ce72a4466fbb27b715c3722558bb15afa9f48
Date: 2021-06-17T13:28:07.755Z
Electron: 12.0.7
Chrome: 89.0.4389.128
Node.js: 14.16.0
V8: 8.9.255.25-electron.0
OS: Windows_NT x64 10.0.19041

To reproduce
Prerequisites:
Extension is already installed and the default CodeQL CLI has been downloaded by the extension already.

1. Open the CodeQL extension settings (@ext:github.vscode-codeql)
2. Change "CodeQL > CLI: Executable Path" to an invalid path
3. Restart VSCode
4. Trigger loading of extensions, e.g. by clicking on CodeQL icon on the left sidebar
   ❌ Notice how neither an error dialog is shown, nor does the "CodeQL" info text in the bottom right corner appear.
   The logs show that the extension failed finding the CodeQL CLI.
   ℹ️ Only in case you had not downloaded CodeQL CLI yet and the executable path is empty it shows the error "The CodeQL CLI could not be found."
5. Run an extension command such as "CodeQL: Quick query"; on the error message click "Install CodeQL CLI"
   ❌ Nothing happens
6. Stop VSCode
7. Open the CodeQL CLI download folder; on Windows %appdata%\Code\User\globalStorage\github.vscode-codeql
8. Delete or rename the distribution folder
9. Start VSCode again
10. Open the CodeQL extension settings (@ext:github.vscode-codeql)
11. Change "CodeQL > CLI: Executable Path" to any non-empty path
    ❌ Instead of checking whether the CodeQL CLI exists at the specified path, the extension starts downloading the default CodeQL CLI

Expected behavior
The extension should properly start even without CodeQL CLI; all the incorrect behavior described above (marked with ❌) should not happen.