Error handling issues in stream processing code in Angular frontend

A few issues have been identified in the stream processing code in the angular frontend:

• JSON-formatted errors received from the server during upload streams are not properly handled, leading to error messages not being displayed to the user
• The list of translatable upload errors is hardcoded in the JS, meaning that translations cannot be supplied for currently untranslated errors by extensions.  (NOTE: the same issue exists for the client notification system)
• In Firefox only, upload streams that are rejected do not contain any response information - the status code is 0 in the XHR, and the response information is blank in the network tab - this is likely related to this issue: https://stackoverflow.com/questions/20210252/status-code-text-and-all-response-headers-are-null-for-an-xhr-that-has-status-c