Improve the error flow for non-initialized project

If I do `amplify add xxx` in a project which hasn't been initialized we currently spit out an ugly trace:

```
authtest $amplify-dev add auth
Scanning for plugins...
Plugin scan successful
You are not working inside a valid amplify project.
Use 'amplify init' in the root of your app directory to initialize your project with Amplify
Error: You are not working inside a valid amplify project.
Use 'amplify init' in the root of your app directory to initialize your project with Amplify
at getAmplifyDirPath (/Users/richardthrelkeld/Downloads/adminauth/amplify-cli/packages/amplify-cli/lib/extensions/amplify-helpers/path-manager.js:49:11)
at getDotConfigDirPath (/Users/richardthrelkeld/Downloads/adminauth/amplify-cli/packages/amplify-cli/lib/extensions/amplify-helpers/path-manager.js:53:37)
at Object.getProjectConfigFilePath (/Users/richardthrelkeld/Downloads/adminauth/amplify-cli/packages/amplify-cli/lib/extensions/amplify-helpers/path-manager.js:81:37)
at AmplifyToolkit.getProjectDetails [as _getProjectDetails] (/Users/richardthrelkeld/Downloads/adminauth/amplify-cli/packages/amplify-cli/lib/extensions/amplify-helpers/get-project-details.js:7:45)
at Object.run (/Users/richardthrelkeld/Downloads/adminauth/amplify-cli/packages/amplify-category-auth/commands/auth/enable.js:15:34)
at Object.executeAmplifyCommand (/Users/richardthrelkeld/Downloads/adminauth/amplify-cli/packages/amplify-category-auth/index.js:318:23)
at /Users/richardthrelkeld/Downloads/adminauth/amplify-cli/packages/amplify-cli/lib/execution-manager.js:114:55
at step (/Users/richardthrelkeld/Downloads/adminauth/amplify-cli/packages/amplify-cli/lib/execution-manager.js:33:23)
at Object.next (/Users/richardthrelkeld/Downloads/adminauth/amplify-cli/packages/amplify-cli/lib/execution-manager.js:14:53)
at fulfilled (/Users/richardthrelkeld/Downloads/adminauth/amplify-cli/packages/amplify-cli/lib/execution-manager.js:5:58)
at
```

Why don't we either:
- Automatically prompt to run amplify init (y/N)
- Remove this stack trace since it's obfuscating the message about running amplify init

Reproduce `amplify add auth` outside an initialized project, then trace the error through `packages/amplify-cli/lib/extensions/amplify-helpers/path-manager.js`, `get-project-details.js`, and `lib/execution-manager.js`. Compare the current output with the issue's proposed init prompt or concise error handling. Done means the user receives clear guidance without an obfuscating stack trace.

Written by the indexing model from the issue text.