Add default handler for uncaught exceptions

[alexdewar]

In general, I think it would be useful to have a way to display to users that an uncaught exception has occurred. If we wanted to be extra fancy, we could have a way to e.g. automatically email the traceback to the devs.

See my implementation here: https://github.com/ImperialCollegeLondon/iPANACEA/blob/8ca614a0b507578a382e08e8cadaf24e38bffe75/ipanacea/extensions/main/presenter.py#L21

(@dalonsoa Maybe this would be another good candidate for Hacktoberfest?)

[dalonsoa]

It's not a bad idea. But, I'd suggest we invest the Hacktoberfest time in stuff we don't have too much opportunity to work as part of a project - eg. side RSE projects like guikit or pycsvy, personal projects, etc. In practice, time permitting, this can be done as part of the activities of the project.