Flask's app.errorhandler decorator dosen't work for GraphQLLocatedError

I am making an extension library [flask-graphql-auth](https://github.com/devArtoria/flask-graphql-auth). and error handler by using flask's app.errorhandler is being planed for the 1.0 release. I checked that if my library caused an exception like JWTDecodeError, an GraphQLLocatedError occurred. However, the flask can not handle the error by app.errorhandler. Is this a bug?

```
@app.errorhandler(GraphQLLocatedError)
def handle(e):
return 401
```
I wrote this code. but stacktrace appears :(
```
...
Traceback (most recent call last):
File "C:\Users\Lewis\OneDrive\Documents\Development Repos\flask-graphql-auth\venv\lib\site-packages\graphql\execution\executor.py", line 330, in complete_value_catching_error
exe_context, return_type, field_asts, info, result)
File "C:\Users\Lewis\OneDrive\Documents\Development Repos\flask-graphql-auth\venv\lib\site-packages\graphql\execution\executor.py", line 383, in complete_value
raise GraphQLLocatedError(field_asts, original_error=result)
graphql.error.located_error.GraphQLLocatedError: Signature verification failed
127.0.0.1 - - [21/Jul/2018 12:48:30] "POST /graphql? HTTP/1.1" 200 -
```

Comece em graphql/execution/executor.py, especialmente em complete_value_catching_error e complete_value, e depois rastreie a requisição /graphql até o registro de app.errorhandler do Flask. Determine por que GraphQLLocatedError ignora o handler reportado; considera-se concluído quando o comportamento do handler estiver coberto de forma reproduzível e a exceção não ignorar mais o tratamento de erros do Flask.

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