Show git error message updating snippet

What does this MR do?

When the snippet update service fails because of the git action we should show that message to the user. At the moment, the controller only shows validation errors but, since this error doesn't belong there, it's not shown.

Refs #39265 (closed)

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