Allow custom errors to be returned from queries, mutations

[pepsighan]

Fixes #40.

This is not a breaking change, as FieldResult still works. For use cases which desire for custom error handling that return additional response fields ergnomically, those error types should implement IntoFieldError trait which converts your error type to the FieldError. This maintains that in the response, message field is always returned along with other optional user desired metadata.

You may implement your error as:

enum CustomError {
  NotFound
}

impl IntoFieldError for CustomError {
  fn into_field_error(self) -> FieldError {
    FieldError::new("Not Found", graphql_value!({ "type": "NOT_FOUND" })
  }
}

And use it likewise:

field add_likes(&executor, id: i32) -> Result<Like, CustomError> {
  Err(CustomError::NotFound)
}

[LegNeato]

Thanks for the change! Have you seen #196 / https://github.com/facebook/graphql/releases/tag/June2018? There are a lot more requirements for errors in the newest spec. Does the newest spec obsolete this PR? For example, graphql/graphql-spec#407.

Direct link to new error section: http://facebook.github.io/graphql/June2018/#sec-Errors

[pepsighan]

@LegNeato I read the error section of the specification and one of the change it outlines is to use extensions field to map out additional metadata to the error. We are currently using data field in its place which needs to be refactored and that could be done in another PR.

As for this PR, it only deals with mapping our own error to the FieldError. So, I think the spec does not have a problem with it.

[LegNeato]

Ok, looks reasonable. Thanks for the test as well. Can you add it to the changelog please?

[LegNeato]

Great, thanks! 🍻 . Sorry this took so long to merge.