DEV: Show tracebacks for failed extensions when running without stdout.

IPython/core/shellapp.py

@@ -278,13 +278,14 b' class InteractiveShellApp(Configurable):'
                     self.log.info("Loading IPython extension: %s" % ext)
                     self.shell.extension_manager.load_extension(ext)
                 except:
-                    self.log.warn("Error in loading extension: %s" % ext +
-                        "\nCheck your config files in %s" % self.profile_dir.location
-                    )
-                    self.shell.showtraceback()
+                    msg = ("Error in loading extension: {ext}\n"
+                           "Check your config files in {location}".format(
+                               ext=ext,
+                               location=self.profile_dir.location
+                           ))
+                    self.log.warn(msg, exc_info=True)
         except:
-            self.log.warn("Unknown error in loading extensions:")
-            self.shell.showtraceback()
+            self.log.warn("Unknown error in loading extensions:", exc_info=True)
 
     def init_code(self):
         """run the pre-flight code, specified via exec_lines"""