Add a dedicated error class for OAuth response parsing errors

[atheriel]

Not all OAuth providers in the wild return spec-compliant responses in the case of an error. A prominent example is Facebook, but I've seen issues with Snowflake OAuth's token exchange flow, too. Posit Connect is not well-behaved here, either.

I've also seen weird issues in the past like authentication failues resulting in redirects to human-readable HTML pages rather than proper OAuth2 error response JSON.

All this to say: I think it can be really useful to have access to the original response object when trying to diagnose an issue.

So this commit adds a dedicated httr2_oauth_parse error class and embeds the response object in it. This allows consumers to write code that handles these errors with a lot more context:

tryCatch(
  some_oauth_using_code(),
  httr2_oauth_parse = function(cnd) {
    cli::cli_abort(
      c(
        "Unexpected failure during an OAuth flow.",
        i = "Status code: {resp_status(cnd$resp)}",
        i = "Response content type: {resp_content_type(cnd$resp)}"
      ),
      parent = cnd
    )
  }
)

Unit tests are included.

[atheriel]

Right now these errors look more like the following:

! Failed to parse response from `client$token_url` OAuth url.
* Did not contain `access_token`, `device_code`, or `error` field.

or

! Unexpected content type "text/plain".
* Expecting type "application/json" or suffix "json".

[hadley]

Thanks!

[jennybc]

... redirects to human-readable HTML pages rather than proper OAuth2 error response JSON

cough, google