Error reporting summaries

[mpizenberg]

While solving all elm packages, I stumbled upon quite a lot of unsolvable packages, for different reasons. I've classified those errors in different categories and I think it would be really nice if the error reporter was able to give one such class of high level problem before diving into the detailed tree. Here are some examples of potential categories:

• Direct/Indirect dependency on a non-existing package
• Indirect dependency on two incompatible versions of the same package
• Indirect dependency on another version of itself

[Eh2406]

The more I thought about this idea the more I love it. These patterns should all be identifiable in a error tree.