Improve error message when logdir/db not given. (#1189)

Author: oxinabox

tensorboard/program.py

@@ -202,8 +202,10 @@ def create_tb_app(plugins, assets_zip_provider=None):
   :rtype: types.FunctionType
   """
   if not FLAGS.db and not FLAGS.logdir:
-    raise ValueError('A logdir must be specified when db is not specified. '
-                     'Run `tensorboard --help` for details and examples.')
+    raise ValueError('A logdir or db must be specified. '
+                     'For example `tensorboard --logdir mylogdir` '
+                     'or `tensorboard --db sqlite:~/.tensorboard.db`. '
+                     'Run `tensorboard --helpfull` for details and examples.')
   return application.standard_tensorboard_wsgi(
       assets_zip_provider=assets_zip_provider,
       db_uri=FLAGS.db,