Adding the ability to show causes in the DebugExceptions view

Hi all,

The Exception#cause method has been added to Ruby 2.1.0 back in 2013. Since then Rails has adopted it to replace the #original_exception method in a few classes such as the ActiveSupport::Rescuable, and I believe we could take a step further to help debug an app in development. One low-hanging fruit that I see is to add the ability to show causes in the DebugExceptions view. I have made a proof-of-concept implementation to see if it could be really useful. Here is a screenshot of what it looks like:

The code could be found here:

Of course, there is going to be more things to take care of, such as implementing more frontend features and updating how a TemplateError (which already uses #cause) could be shown, but I wanted to get feedback (especially on the visual aspect of the PoC) before I invest more time in it.

Let me know what you all think.



Looks good to me. Please open a pr