Handle streaming errors like regular errors

[hadley]

Fixes #479

@jwimberl how does this look to you?

[jwimberl]

Testing this branch, I see uniform behavior btw req_perform and req_perform_stream in one of the reprex cases from #479:

packageVersion("httr2")
#> [1] '1.0.1.9000'
library(httr2)
req <- request("https://api.github.com/asdfsdadf")
a <- req |> req_perform()
#> Error in `req_perform()`:
#> ! HTTP 404 Not Found.
last_response() |> resp_body_string()
#> [1] "{\n  \"message\": \"Not Found\",\n  \"documentation_url\": \"https://docs.github.com/rest\",\n  \"status\": \"404\"\n}\n"
b <- req |> req_perform_stream(callback = \(z) TRUE)
#> Error in `req_perform_stream()`:
#> ! HTTP 404 Not Found.
last_response() |> resp_body_string()
#> [1] "{\n  \"message\": \"Not Found\",\n  \"documentation_url\": \"https://docs.github.com/rest\",\n  \"status\": \"404\"\n}\n"

^{Created on 2024-07-12 with reprex v2.1.1}

So, both lead to an R error, and last_response() contains the error body in both cases, which looks good to me, as this most likely how I would want to call endpoints like this.

I do see a slight difference behavior in the other reprex case:

packageVersion("httr2")
#> [1] '1.0.1.9000'
library(httr2)
req <- request("https://api.github.com/asdfsdadf")
a <- req |> req_error(is_error = \(z) FALSE) |> req_perform()
last_response() |> resp_body_string()
#> [1] "{\n  \"message\": \"Not Found\",\n  \"documentation_url\": \"https://docs.github.com/rest\",\n  \"status\": \"404\"\n}\n"
b <- req |> req_error(is_error = \(z) FALSE) |> req_perform_stream(callback = \(z) TRUE)
last_response() |> resp_body_string()
#> Error in `resp_body_raw()`:
#> ! Can't retrieve empty body.

^{Created on 2024-07-12 with reprex v2.1.1}

but this makes sense too -- if I'm overriding things to tell it its not an error, of course it would leave the body to be handled by the callback. So, this seems consistent to me as well.

[hadley]

@jwimberl thanks for the thoughtful review. I really appreciate it 😄