Fix snippet repository storage move API when snippet does not exist

What does this MR do?

In the snippet repository storage move API, we are not checking whether the snippet exists or not. This means that, if it doesn't exist, we end up in a 500 error.

In this MR we place a check to ensure that it exists, otherwise the response ends with a 404.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content