Handle failure to fetch a document in the DocViewer

[pmarsh-scottlogic]

Suppose you open the <DocumentViewBox>, and the backend decides to misbehave and not return documents. We have already handled the case where we cannot load the documentMetas #712. But what if we have already loaded the documentMetas and then the backend fails? Well, then we are presented with a blank doc viewer. We should put some kind of error message there.

To reproduce:

• Run the frontend and backend
• In sandbox, click "view documents" to open the document vieweer
• shut down the backend
• try loading up the next document

You might need to get into the guts of the library component <DocViewer> from '@cyntler/react-doc-viewer'