[ENH] Better error handling when submitting bots

[Aran30]

1. Motivation - If something is wrong with your model when submitting the bot, or if any other error occurs while submitting the model, you don't get any concrete explanation on what happened. The only thing the users see, is "Sending failed". Making it hard to understand what exactly went wrong, making viewing the logs neccessary...
2. Specification - Upgrade the error messages in the frontend and create more specific error explanations in the backend.
3. Finalised state - People using the frontend have a better grasp on what is wrong with their model.

Note: Changes need to take place in both frontend and backend.