Fix error formatting in debug mode

Backtrace filtering was only disabled for exception cause, this change
ensure no backtrace filtering at all in debug mode.

* Prevent main exception filtering when debug mode is enabled;
* Test when exception cause must be displayed;
* Test how exception cause must be displayed.

lib/producer/core/error_formatter.rb

@@ -18,7 +18,7 @@ module Producer
       end
 
       def format(exception)
-        lines = format_exception exception
+        lines = format_exception exception, filter: !debug?
 
         if debug? && exception.cause
           lines << ''