Exception traceback disappears when running middleware

Here's a small example that illustrates the issue:

```python
import logging

import graphene

logging.basicConfig()

class Query(graphene.ObjectType):
hello = graphene.String()

def resolve_hello(self, info):
raise Exception('error')
return 'Hello'

schema = graphene.Schema(query=Query)

def middleware(next, root, info, **kwargs):
return next(root, info, **kwargs)
```

If we execute a query without any middleware, we get the traceback:

```
In [3]: schema.execute('{ hello }')
ERROR:graphql.execution.executor:An error occurred while resolving field Query.hello
Traceback (most recent call last):
File "/home/felipe/.local/share/virtualenvs/graphene-hello-viSdSxbU/local/lib/python2.7/site-packages/graphql/execution/executor.py", line 447, in resolve_or_error
return executor.execute(resolve_fn, source, info, **args)
File "/home/felipe/.local/share/virtualenvs/graphene-hello-viSdSxbU/local/lib/python2.7/site-packages/graphql/execution/executors/sync.py", line 16, in execute
return fn(*args, **kwargs)
File "", line 12, in resolve_hello
raise Exception('error')
Exception: error
ERROR:graphql.execution.utils:Traceback (most recent call last):
File "/home/felipe/.local/share/virtualenvs/graphene-hello-viSdSxbU/local/lib/python2.7/site-packages/graphql/execution/executor.py", line 447, in resolve_or_error
return executor.execute(resolve_fn, source, info, **args)
File "/home/felipe/.local/share/virtualenvs/graphene-hello-viSdSxbU/local/lib/python2.7/site-packages/graphql/execution/executors/sync.py", line 16, in execute
return fn(*args, **kwargs)
File "", line 12, in resolve_hello
raise Exception('error')
GraphQLLocatedError: error
```

But if we run it with the middleware, we get very little information:

```
In [4]: schema.execute('{ hello }', middleware=[middleware])
ERROR:graphql.execution.utils:GraphQLLocatedError: error
```

While investigating the issue, it looks like the point the traceback gets lost is around https://github.com/graphql-python/graphql-core/blob/master/graphql/execution/executor.py#L529. The promise gets rejected because of the exception, and then the `is_reject` handler returns *another* promise that gets immediately rejected. This new error has no traceback, so it doesn't get logged when it's caught inside `complete_value_catching_error`.

Start with graphql/execution/executor.py around line 529 and follow the promise rejection path into complete_value_catching_error. Reproduce the supplied schema and middleware example, comparing execution with and without middleware. Done means the middleware case retains and logs the resolver traceback rather than only GraphQLLocatedError.

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