Feature: handle revoked API access through Figshare

Purpose
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Addresses CenterForOpenScience/openscienceframework.org#332.

When a user revokes OSF access to the Figshare addon through Figshare, the linked files do not show in HGrid and no error messages are shown indicating that their addon credentials are no longer valid. 

Changes
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- Show figshare folder that was previously linked in HGrid (with no files) and throw an error message informing the user that figshare cannot be reached.
- Show error messages on node and user settings page saying that figshare credentials are no longer valid