Handle users who have no index.json

[Hopding]

Description

This PR causes an error to be thrown if a GET for a user's index.json returns a response containing this xml:

<Error><Code>NoSuchKey</Code><Message>The specified key does not exist.</Message>

and the user's snippet list menu option is disabled until they possess snippet(s) & an index file.

Motivation and Context

Fixes #411. This PR solves the problem of users with no index.json getting an infinite empty list of snippets.

Checklist:

• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

Screenshots:

User with no snippets

User with snippets