enhancement: DjangoDebugContext captures exceptions

[zbyte64]

DjangoDebugContext captures exceptions and allows captured stack traces to be queried. Example usage added to documentation.

Fixes #1121

Discovered that next does not return a promise in our test environment. Do I need to check if the return is a promise?

[tcleonard]

Awesome! Was looking for that for a while!

[tcleonard]

Discovered that next does not return a promise in our test environment. Do I need to check if the return is a promise?

Just saw that comment... not sure why next does not return a promise in the first place, but what is the implication of this?

[zbyte64]

Could mean an exception might get raised and not logged. Or maybe the variable name is incorrect. My current understanding is it gets call by this: https://github.com/graphql-python/graphql-core/blob/6494c7fcfa62418a4fe36cf23e0bbfebc10fe342/src/graphql/execution/middleware.py#L36 which doesn't seem to do anything with promises...

[khink]

The try-except around promise = next(root, info, **args) in e9f25ec is the cause of issue #1367. It makes raising a GraphQLError return a "Promise rejected" instead of an "errors" list. From what i can gather, raising GraphQLError is a pattern that is supported throughout Graphene.

If the try-except could be removed, that would make graphene-django work more as people might expect, i think.