Add default messages to SanicExceptions (#2216)

* Add default messages to SanicExceptions

* Cleaner exception message setting

sanic/exceptions.py

@@ -4,16 +4,20 @@ from sanic.helpers import STATUS_CODES
 
 
 class SanicException(Exception):
+    message: str = ""
+
     def __init__(
         self,
         message: Optional[Union[str, bytes]] = None,
         status_code: Optional[int] = None,
         quiet: Optional[bool] = None,
     ) -> None:
-
-        if message is None and status_code is not None:
-            msg: bytes = STATUS_CODES.get(status_code, b"")
-            message = msg.decode("utf8")
+        if message is None:
+            if self.message:
+                message = self.message
+            elif status_code is not None:
+                msg: bytes = STATUS_CODES.get(status_code, b"")
+                message = msg.decode("utf8")
 
         super().__init__(message)
 

tests/test_exceptions.py

@@ -235,6 +235,22 @@ def test_sanic_exception(exception_app):
     assert len(w) == 1 and "deprecated" in w[0].message.args[0]
 
 
+def test_custom_exception_default_message(exception_app):
+    class TeaError(SanicException):
+        message = "Tempest in a teapot"
+        status_code = 418
+
+    exception_app.router.reset()
+
+    @exception_app.get("/tempest")
+    def tempest(_):
+        raise TeaError
+
+    _, response = exception_app.test_client.get("/tempest", debug=True)
+    assert response.status == 418
+    assert b"Tempest in a teapot" in response.body
+
+
 def test_exception_in_ws_logged(caplog):
     app = Sanic(__file__)