Add error argument to req_perform_iteratively()

[mgirlich]

In case a request fails it might still be interesting to continue (if possible) with the pagination or return the intermediate results.

[hadley]

There's possibly some connection to cancel_on_error on in multi_req_perform(). We should probably also switch to using an enum for the reasons mentioned in https://design.tidyverse.org/explicit-strategies.html#only-two-strategies.

[mgirlich]

The error message when parse_resp() fails should also be improved.

[hadley]

Maybe something like:

• "error" (the default): the iteration ends with an error.
• "terminate": terminate the iteration and return progress
• "continue": continue, ignoring the error

If you use terminate or continue the return value might be a mix of responses and errors, not just responses.

[mgirlich]

Sounds good to me.

[hadley]

I realised continue won't work for most of the iteration helpers (i.e. anything that depends on a request), and none of the error behaviours is terribly useful if we don't have a way to also track the response. I liked your idea of adding the request as attribute to the response, and maybe it makes sense to do that for the httr2_failure and httr2_http errors too.