fix(@angular/cli): explain the latest version fallback when ng add finds no compatible version

When no version of the package satisfies the peer dependencies of the project,
`ng add` reports `Unable to find compatible package.` and then continues to
install the version of the `latest` tag. The explanation that the `latest` tag is
used was dropped when the command was migrated to the new package manager
abstraction, so the installation that follows the message is unexpected.

Restore the `Using 'latest' tag.` explanation.

Author: MeAkib

packages/angular/cli/src/commands/add/cli.ts

@@ -422,7 +422,7 @@ export default class AddCommandModule
     }
 
     if (!found) {
-      let message = `Unable to find compatible package.`;
+      let message = `Unable to find compatible package. Using 'latest' tag.`;
       if (rejectionReasons.length > 0) {
         message +=
           '\nThis is often because of incompatible peer dependencies.\n' +