ENH: changes the behavior of delete_parameter to respond with a 400 error when the parameter does not exist.

Currently, the delete_parameter function for the ssm client will respond with a dict containing a key of Invalid Parameter which has a value of a list containing the parameter name that was requested to be deleted when a parameter with said name doesn't exist which doesn't match the behavior of boto3.

moto/ssm/models.py

@@ -278,10 +278,7 @@ class SimpleSystemManagerBackend(BaseBackend):
                 self._region = region
 
     def delete_parameter(self, name):
-        try:
-            del self._parameters[name]
-        except KeyError:
-            pass
+        return self._parameters.pop(name, None)
 
     def delete_parameters(self, names):
         result = []

moto/ssm/responses.py

@@ -22,7 +22,13 @@ class SimpleSystemManagerResponse(BaseResponse):
 
     def delete_parameter(self):
         name = self._get_param("Name")
-        self.ssm_backend.delete_parameter(name)
+        result = self.ssm_backend.delete_parameter(name)
+        if result is None:
+            error = {
+                "__type": "ParameterNotFound",
+                "message": "Parameter {0} not found.".format(name),
+            }
+            return json.dumps(error), dict(status=400)
         return json.dumps({})
 
     def delete_parameters(self):