Better error handling

[langdal]

When the backend encounters errors the user is only notified by a generic "An error happened" dialog with no way of debugging what went wrong.

A possible solution could be to include the complete stack trace from python in the result and expose it to the user through some kind of "further details" link in the error dialog.

When an error occurs it should be represented in a persistent manner, e.g., not closing the toast automatically or adding an indication to the "run" button

Let's discuss whether the json should carry a field with the std-out from running the python. This would catch some error massages that could be useful in bug-fixing.

In longer terms, this leads to some questions related to whether the json should carry some sort of history for what has been done during the opt.

Handling errors during backend communication is dependent on the resolution of #150