🐛 `file_progress` error report sends object with exception

[AlbertMN]

When an error occurs during, or after, the file download process, we send {'type': 'file_progress', 'data': {'state': 'error', 'message': 'text'}, but also with an exception key, which can hold an exception-class rather than just the messsage.

Example from log file;

{'type': 'file_progress', 'data': {'state': 'error', 'message': 'Error processing download', 'exception': StorageError('cr10-shell-nctest.nc is an unrecognized file type')}}

This never makes it to the server, and I'm guessing it's because we can't serialize StorageError to proper JSON that can be sent or received.

I propose merging "message" with any potential exception messages, so that it's; 'message': 'Error processing download: {error message from exception as a string}