[hosted plugin] fix no indication for selecting an invalid node.js project

[DoroNahari]

What it does

When selecting a node.js project that is not a valid plugin(vscode ext or theia plugin) for hosted plugin instance, it failed selecting the project as expected but doesn't notify the user.

In the hosted-plugin-manager-client.js it checks if the plugin is valid. then the isPluginValid function in hosted-instance-manger.js (backend) calling to getScanner which throws an error because there's no valid scanner for this project.

I wrote a code to catch this error in the isPluginValid function and return false to the client accordingly.

Before:

After:

How to test

Clone node.js project which is not a vscode extension or theia plugin.
Press f1 -> Hosted Plugin: Select Path -> select the project.
See that there's a notification telling the user that the folder is not a valid plugin.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[amiramw]

Code looks good to me and tested on git pod, the alert is shown as expected.

[RomanNikitenko]

Looks good to me.
Tested, it works well for me!