title: supress_callback_exceptions = True no loger works
body: <p>In an older version of <code>Dash</code>, I used to need to be able to say, </p>
  
  <pre><code>supress_callback_exceptions = True
  </code></pre>
  
  <p>Since I upgraded to the new version of <code>Dash</code>, now I get an error on that line:</p>
  
  <pre><code>Traceback (most recent call last):
    File "dash-yahoo_options.py", line 2109, in &lt;module&gt;
      app.config.supress_callback_exceptions = True
    File "/home/idf/anaconda3/envs/dash_apps_36/lib/python3.7/site-packages/dash/_utils.py", line 81, in __setattr__
      self[key] = value
    File "/home/idf/anaconda3/envs/dash_apps_36/lib/python3.7/site-packages/dash/_utils.py", line 106, in __setitem__
      raise AttributeError(final_msg, key)
  AttributeError: ('Invalid config key. Some settings are only available via the Dash constructor', 'supress_callback_exceptions')
  (dash_apps_36) idf@ubvm:~/Downloads$ vi dash-yahoo_options.py
  (dash_apps_36) idf@ubvm:~/Downloads$ python dash-yahoo_options.py
  Traceback (most recent call last):
    File "dash-yahoo_options.py", line 2109, in &lt;module&gt;
      app.config.supress_callback_exceptions = True
    File "/home/idf/anaconda3/envs/dash_apps_36/lib/python3.7/site-packages/dash/_utils.py", line 81, in __setattr__
      self[key] = value
    File "/home/idf/anaconda3/envs/dash_apps_36/lib/python3.7/site-packages/dash/_utils.py", line 106, in __setitem__
      raise AttributeError(final_msg, key)
  AttributeError: ('Invalid config key. Some settings are only available via the Dash constructor', 'supress_callback_exceptions')
  </code></pre>