Fix bug with snippet updating error losing changes

What does this MR do?

Refs #217561 (closed)

Changes the Snippets update service to re-assign any changes when an error occurs during update, so that these changes are not lost for the user.

Previous to this MR, if a repository error occured during update, the previous attributes would be restored on the snippet which would remove any changes to content etc that the user may have made

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.