Use Exception.message/1 instead of exception.message
Update CHANGELOG
Soft deprecate is_exception/1 in favor of Exception.exception?/1
Convert ExUnit.Test and ExUnit.TestCase to structs
Improve application error formatting

* Format (nested) supervision tree init failures
* Format :gen_server exit reasons
* Fixes :invalid_options text
* Fixes text when when application start returns {:error, reason}