Add handling of failed publish.

[nagolucky18]

When publishing, if the selected page does not yet have a page created, handle the FailedPublishNothingToPublish error so that the user is not presented with an exception. A warning is shown to the user that some languages failed to publish due to nothing to publish and to check that a page has been created for selected languages. Additionally fixed a validation issue where publish would always succeed if all languages were selected.

Issue: 15352

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes 15352

Description

This pull request contains the following changes:

• Handle the FailedPublishNothingtoPublish publish result in ContentController.cs. A warning now appears when the user attempts to publish a language for which no page has been created. See below image for warning text contents.

• Removed conditions to always publish successfully if all languages are selected.

Steps to test (taken from main issue body):

1. Create a page with List view enabled.
2. Enable several languages in the settings section.
3. Create a child page and set "Allow vary by culture" to true. This page should be created in only one language.
4. Go back to the List view page.
5. Select the created page.
6. Click "Publish" in top-right.
7. Select a language for which a page has not been created.
8. Click "Publish" in the popup.
9. Observe that a warning message appears informing the user that a publish was attempted for a page that has not been created.
10. Repeat 7. but select all languages.
11. Observe warning message from 9.

[github-actions]

Hi there @nagolucky18, thank you for this contribution! 👍

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[busrasengul]

Thank you @nagolucky18
Someone from the Core collabs team will look into this! Mind that it's holiday times so might take some time :)
Merry Chrismas 🎉

[busrasengul]

Let's get this merge in :) 🎉