add default type key on ExInfo errors to distinguish them from other ExInfo

Right now it adds a bit of boilerplate to deal with validation error that pop via ExInfo in an app that makes heavy use of them already. Adding a simple :type key allows to distinguish them from others efficiently.

This patch adds {:type :schema/error ..} on the ExInfo map created via the error! macro.
It's possible to overwrite this value via error! if a :type key is supplied to the map arg.