title: Flask fails with "Error: While importing 'X', an ImportError was raised", but does not display the error. How to find the source of the error?
body: <p>When starting a Flask app with:</p>
  <pre class="lang-sh prettyprint-override"><code>$ flask run
  </code></pre>
  <p>I received the error:</p>
  <blockquote>
  <pre><code>Error: While importing 'wsgi', an ImportError was raised.
  
  Usage: flask [OPTIONS] COMMAND [ARGS]...`
  ...
  </code></pre>
  </blockquote>
  <p>However, there is no stack trace or other information provided. What is the best way to get the <code>ImportError</code> stack trace?</p>