Add show_tracebacks trait

Author: vidartf

voila/app.py

@@ -76,7 +76,10 @@ class Voila(Application):
     examples = 'voila example.ipynb --port 8888'
 
     flags = {
-        'debug': ({'Voila': {'log_level': logging.DEBUG}}, _("Set the log level to logging.DEBUG")),
+        'debug': (
+            {'Voila': {'log_level': logging.DEBUG, 'show_tracebacks': True}},
+            _("Set the log level to logging.DEBUG, and show exception tracebacks in output.")
+        ),
         'no-browser': ({'Voila': {'open_browser': False}}, _('Don\'t open the notebook in a browser after startup.'))
     }
 
@@ -186,6 +189,10 @@ class Voila(Application):
         )
     )
 
+    show_tracebacks = Bool(False, config=True, help=_(
+        'Whether to send tracebacks to clients on exceptions.'
+    ))
+
     port_retries = Integer(50, config=True,
                            help=_("The number of additional ports to try if the specified port is not available.")
                            )

voila/execute.py

@@ -79,7 +79,7 @@ async def execute_cell(self, cell, resources, cell_index, store_history=True):
 
     def should_strip_error(self):
         """Return True if errors should be stripped from the Notebook, False otherwise, depending on the current config."""
-        return 'Voila' not in self.config or 'log_level' not in self.config['Voila'] or self.config['Voila']['log_level'] != logging.DEBUG
+        return 'Voila' not in self.config or not self.config['Voila'].get('show_tracebacks', False)
 
     def strip_notebook_errors(self, nb):
         """Strip error messages and traceback from a Notebook."""