Before, returning a `bool` from a route caused the error
```
[2019-05-31 10:08:42,216] ERROR in app: Exception on / [GET]
Traceback (most recent call last):
File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 2070, in make_response
rv = self.response_class.force_type(rv, request.environ)
File "/Users/johnzeringue/Documents/ts-open/flask/env/lib/python3.7/site-packages/werkzeug/wrappers/base_response.py", line 269, in force_type
response = BaseResponse(*_run_wsgi_app(response, environ))
File "/Users/johnzeringue/Documents/ts-open/flask/env/lib/python3.7/site-packages/werkzeug/wrappers/base_response.py", line 26, in _run_wsgi_app
return _run_wsgi_app(*args)
File "/Users/johnzeringue/Documents/ts-open/flask/env/lib/python3.7/site-packages/werkzeug/test.py", line 1119, in run_wsgi_app
app_rv = app(environ, start_response)
TypeError: 'bool' object is not callable
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 2393, in wsgi_app
response = self.full_dispatch_request()
File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 1906, in full_dispatch_request
return self.finalize_request(rv)
File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 1921, in finalize_request
response = self.make_response(rv)
File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 2078, in make_response
reraise(TypeError, new_error, sys.exc_info()[2])
File "/Users/johnzeringue/Documents/ts-open/flask/flask/_compat.py", line 39, in reraise
raise value.with_traceback(tb)
File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 2070, in make_response
rv = self.response_class.force_type(rv, request.environ)
File "/Users/johnzeringue/Documents/ts-open/flask/env/lib/python3.7/site-packages/werkzeug/wrappers/base_response.py", line 269, in force_type
response = BaseResponse(*_run_wsgi_app(response, environ))
File "/Users/johnzeringue/Documents/ts-open/flask/env/lib/python3.7/site-packages/werkzeug/wrappers/base_response.py", line 26, in _run_wsgi_app
return _run_wsgi_app(*args)
File "/Users/johnzeringue/Documents/ts-open/flask/env/lib/python3.7/site-packages/werkzeug/test.py", line 1119, in run_wsgi_app
app_rv = app(environ, start_response)
TypeError: 'bool' object is not callable
The view function did not return a valid response. The return type must be a string, tuple, Response instance, or WSGI callable, but it was a bool.
```
Now, it returns the more readable
```
[2019-05-31 10:36:19,500] ERROR in app: Exception on / [GET]
Traceback (most recent call last):
File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 2400, in wsgi_app
response = self.full_dispatch_request()
File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 1907, in full_dispatch_request
return self.finalize_request(rv)
File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 1922, in finalize_request
response = self.make_response(rv)
File "/Users/johnzeringue/Documents/ts-open/flask/flask/app.py", line 2085, in make_response
" {rv.__class__.__name__}.".format(rv=rv))
TypeError: The view function did not return a valid response. The return type must be a string, dict, tuple, Response instance, or WSGI callable, but it was a bool.
```
Fixes#3214
8bb7185284