GH-4292: GSP - Correctly determine graph existence

[afs]

GitHub issue resolved #4292

Pull request Description:

• Fix GSP DELETE on a missing graph returns 404 but also creates an empty graph #4292
• Deal with compiler warnings from earlier PRs
• Use Locale.ROOT for object formatting so that the server location isn't (indirectly) exposed

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• Tests are included.
• Commits have been squashed to remove intermediate development commit messages.
• Key commit messages start with the issue number (GH-xxxx)

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