Uncaught upload errors

e.g.

Failed to load resource: the server responded with a status of 413 () cshare.34b6cc472f19ae772443.js:1 Uncaught (in promise) Error: HTTP status 413 at cshare.34b6cc472f19ae772443.js:1:4062

which corresponds to the "if (!response.ok) throw new Error()" line in upload() in transfer.js