vauth: Use CURLE_AUTH_ERROR for auth function errors

- Add new error code CURLE_AUTH_ERROR.

Prior to this change auth function errors were signaled by
CURLE_OUT_OF_MEMORY and CURLE_RECV_ERROR, and neither one was
technically correct.

Ref: https://github.com/curl/curl/pull/3848

Co-authored-by: Dominik Hölzl

Closes https://github.com/curl/curl/pull/3864

docs/libcurl/libcurl-errors.3

@@ -254,6 +254,8 @@ Status returned failure when asked with \fICURLOPT_SSL_VERIFYSTATUS(3)\fP.
 Stream error in the HTTP/2 framing layer.
 .IP "CURLE_RECURSIVE_API_CALL (93)"
 An API function was called from inside a callback.
+.IP "CURLE_AUTH_ERROR (94)"
+An authentication function returned an error.
 .IP "CURLE_OBSOLETE*"
 These error codes will never be returned. They were used in an old libcurl
 version and are currently unused.

docs/libcurl/symbols-in-versions

@@ -39,6 +39,7 @@ CURLCLOSEPOLICY_SLOWEST         7.7
 CURLE_ABORTED_BY_CALLBACK       7.1
 CURLE_AGAIN                     7.18.2
 CURLE_ALREADY_COMPLETE          7.7.2
+CURLE_AUTH_ERROR                7.66.0
 CURLE_BAD_CALLING_ORDER         7.1           7.17.0
 CURLE_BAD_CONTENT_ENCODING      7.10
 CURLE_BAD_DOWNLOAD_RESUME       7.10