"This should not happen!" is a stupid error message

I tried installing something from an online repository. That errored out while downloading. In the resulting messagebox I clicked "Retry" which resulted in a qfatal with the message "This should not happen!".

This has got something to do with invalid values for the progressbar.

a) Improve the error message, so that it tells us something about what actually was wrong.
b) Don't make this fatal.