Improve UI error handling

[JulienVig]

Is your feature request related to a problem? Please describe.
There is currently no user feedback whenever a feature breaks in the UI, which makes it unclear when something is working as expected or not. Detecting a crash requires one to inspect the console logs from the browser, which is not appropriate for all users.

Describe the solution you'd like
Include more error handling to inform the user whenever something went wrong, as it is already the case for some parts of the website. For example, trying to train a model without first uploading data creates a toaster (pop-up) saying "No file uploaded". I would like to implement something similar covering all errors.

[JulienVig]

Here are some examples of error handling with user feedback:

When uploading data and the error source has been identified:

Or a more generic one when the error source isn't known: