title: flask doesn't register an exception
body: <p>here is my exception class 
      DB_ERROR_REMOVE = 203</p>
  
  <pre><code>CLOUD_ERROR = 300
  
  class MyError(Exception):
      def __init__(self, code, message, recommendation):
          self.code = code
          self.message = message
          self.recommendation = recommendation
          Exception.__init__(self)
  </code></pre>
  
  <p>here is the flask init code:</p>
  
  <pre><code>app = Flask(__name__)
  api = Api(app)
  
  @app.errorhandler(MyError)
  def my_exception_handler(error):
      return "!!!!"  + repr(error) + repr(traceback.format_exc())
  </code></pre>
  
  <p>and I'm raising my error like so:</p>
  
  <pre><code>raise MyError(1,"bla","foo")    
  </code></pre>
  
  <p>doesn't catch it. 
  if I do the same with Exception, it indeed catches it:</p>
  
  <pre><code>@app.errorhandler(Exception)
  def my_exception_handler(error):
      return "!!!!"  + repr(error) + repr(traceback.format_exc())
  </code></pre>