Add reason argument to set_status() so that custom messages flow

Without adding a named argument of `reason` to tornado's `set_status()`
method, messages associated with a thrown HTTPError will not be returned
to the caller.  Instead, the reason corresponding to the built-in status
code is returned.  In addition, because the reason argument is used,
enforcement of a built in status code is skipped, thereby allowing for
the optional use of non-builtin status codes.

Fixes #283

Author: kevin-bates

.gitignore

@@ -56,5 +56,8 @@ docs/_build/
 # PyBuilder
 target/
 
+# PyCharm
+.idea/
+
 .DS_Store
 .ipynb_checkpoints/

kernel_gateway/mixins.py

@@ -125,5 +125,5 @@ def write_error(self, status_code, **kwargs):
                 reply['reason'] = custom_reason
 
         self.set_header('Content-Type', 'application/json')
-        self.set_status(status_code)
+        self.set_status(status_code, reason=reply['reason'])
         self.finish(json.dumps(reply))