Add a clearer panic message if settings.json is malformed

[cristian-paris]

This commit aims to provide a clearer error message when attempting to install the available tools using the sc-meta install all command, in the rare case when a user has a malformed VSCode settings.json file.

For example, let's consider these two settings.json file examples:

1. Correct

{
  "workbench.iconTheme": "vscode-icons",
  "[typescript]": {
    "editor.formatOnSave": true,
    "editor.defaultFormatter": "vscode.typescript-language-features"
  }
}

2. Wrong

{
  "workbench.iconTheme": "vscode-icons",
  "[typescript]": {
    "editor.formatOnSave": true,
    "editor.defaultFormatter": "vscode.typescript-language-features",
  }
}

In the second case we have a trailling comma, after the end of the editor.defaultFormatter row. While this additional comma won't be a problem for VSCode to parse the settings file, this is inherently a malformed json, if we verify it with any online linter. This, in turn, will cause problems for serde. The fix is to just instruct the user to perform a lint check of its settings file, and then proceed with the installation.