Log original exception in debug mode.

Sanic is collecting the information of original exceptions but does
not print it on console. Because it is very useful to debug user code,
it is better to be logged as debug logs.

sanic/sanic.py

@@ -217,6 +217,14 @@ class Sanic:
             # -------------------------------------------- #
             # Response Generation Failed
             # -------------------------------------------- #
+            if self.debug:
+                error_body = response.body
+                if isinstance(error_body, bytes):
+                    try:
+                        error_body = error_body.decode()
+                    except:
+                        pass
+                log.debug(error_body)
 
             try:
                 response = self.error_handler.response(request, e)