Add show_tracebacks trait

Author: vidartf

voila/app.py

@@ -76,7 +76,7 @@ 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")),
         'no-browser': ({'Voila': {'open_browser': False}}, _('Don\'t open the notebook in a browser after startup.'))
     }
 
@@ -186,6 +186,10 @@ 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

@@ -32,7 +32,7 @@ def strip_code_cell_warnings(cell):
 
 def should_strip_error(config):
     """Return True if errors should be stripped from the Notebook, False otherwise, depending on the current config."""
-    return 'Voila' not in config or 'log_level' not in config['Voila'] or config['Voila']['log_level'] != logging.DEBUG
+    return 'Voila' not in config or config['Voila'].get('show_tracebacks', False)
 
 
 class VoilaExecutor(NotebookClient):