Fix operations dashboard flash error message

What does this MR do?

Adds the project name to the flash message displayed when a project cannot be added to the operations dashboard.

Before this change, clicking the Add Projects button would trigger the action for projects to be added, but it would also trigger the modal to close, which would clear the list of selected projects before the flash message could be rendered.

Now the list of selected projects is cleared after the projects are added, allowing the modal to be shown first, with the names of the invalid projects. And the list is also cleared when the modal is canceled.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60307

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers