Check if a file storage file or directory already exists with same key

[apburnes]

When a user uploads a file or creates a directory, notify them the file or directory already exists with the same name.

Notes

• Should run a server side check on the two POST request endpoints
• Possibly create an endpoint that checks if file exists before a user attempts to upload file or create directory

Acceptance Criteria

• Add checks to the file-storage service
• Return JSON message from file-storage service notifying the user of a conflict
• Create api endpoint that check if file/directory with matches with same key