Drop "traceback" from ipynb_error field

The traceback and message end up being the same; so we'll only show the
"message" for now. We can re-add the "traceback" later if we find it
does contain more information.

src/noteburst/handlers/v1/models.py

@@ -31,7 +31,6 @@
 class NotebookError(BaseModel):
     """Information about an exception that occurred during notebook exec."""
 
-    traceback: str = Field(description="The traceback of the exception.")
     name: str = Field(description="The name of the exception.")
     message: str = Field(description="The exception's message.")
 
@@ -43,7 +42,6 @@ def from_nbexec_error(
         is the result of execution in ``/user/:username/rubin/execute``.
         """
         return cls(
-            traceback=error.traceback,
             name=error.ename,
             message=error.err_msg,
         )