#7141787 Plugins: Improve messaging for Jetpack Scan detected errors

When uploading a plugin, it is scanned using Jetpack. The previous message
was unclear, so it has been improved to a more explicit message.

Depends on D74922-code

Changes proposed in this Pull Request

• Change the error message when Jetpack Scan reports a failure from

  Upload problem: Invalid plugin.

  to

  The uploaded file is identified as malicious.

Additionally, the displayed page offers a workaround that reads:

This plugin is identified as malicious. If you want to install the plugin, please continue by uploading the plugin again from WP Admin.

Before	After

Pinging to @emmnyc85 and @cavalierlife to check if the suggested message is fine 🙂

Testing instructions

• Apply D74922-code
• In an Atomic site, go to https://wordpress.com/plugins/upload
• Upload a plugin that fails, for example, this one pdh6GB-6h-p2#comment-698
• The notification message should display The uploaded file is identified as malicious. and the displayed page should contain the message This plugin is identified as malicious. If you want to install the plugin, please continue by uploading the plugin again from WP Admin.. That page will contain two action buttons:
  • Back that will navigate to previous page
  • Continue that will re-direct to the upload plugin from the wp-admin page.

Fixes #58942