Handle snippet file path errors in services

What does this MR do?

Refs #216273 (closed)

Now that we handle Gitaly path errors when creating/updating snippets, we should relay that error message to the user so that they may rectify the issue. Otherwise they'll be presented with a generic message and not know how to proceed.

This MR handles that by refactoring the create and update services.

As agreed with @mnichols1, I've also modified the copy to read Error [creating/updating] the snippet [- optional extra detail], as below:

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.