pyee expects an exception when emitting an error

See the following traceback.
```
10.x.x.x - - [27/Jun/2018 08:13:45] "POST /slack/events HTTP/1.1" 500 -
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 2309, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 2295, in wsgi_app
    response = self.handle_exception(e)
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1741, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 2292, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1815, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1718, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1813, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1799, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python3.6/site-packages/slackeventsapi/server.py", line 63, in event
    self.emitter.emit('error', 'invalid verification token')
  File "/usr/local/lib/python3.6/site-packages/pyee/__init__.py", line 178, in emit
    raise args[0]
TypeError: exceptions must derive from BaseException
```

Author: psykzz

slackeventsapi/server.py

@@ -60,7 +60,7 @@ def event():
             # Verify the request token
             request_token = event_data.get("token")
             if self.verification_token != request_token:
-                self.emitter.emit('error', 'invalid verification token')
+                self.emitter.emit('error', Exception('invalid verification token''))
                 return make_response("Request contains invalid Slack verification token", 403)
 
             # Parse the Event payload and emit the event to the event listener