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

還沒有人認領這個 Issue。

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 -
```

從 graphql/execution/executor.py 開始，特別查看 complete_value_catching_error 和 complete_value，然後追蹤 /graphql 請求直到 Flask 的 app.errorhandler 註冊處。釐清 GraphQLLocatedError 為什麼會繞過所回報的 handler；完成的標準是 handler 行為受到可重現的涵蓋，且該 exception 不再繞過 Flask 錯誤處理。

由索引模型根據 Issue 內容生成。