Resolve "Bug when Snippet creation fails"

What does this MR do?

Stops the Snippet::CreateService from returning the deleted snippet to the caller (Personal/Project Snippet controllers) when it encounters an error.

This is because it trips up the path helpers when re-rendering the new snippet form, to generate urls such as snippets.123 or project-name/snippets.123.

This change re-assigns a snippet instance variable to be a duplicate of the original, so it has everything the controller needs except it won't have the old snippet ID.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

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

Closes #212223 (closed)