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

include/curl/curl.h

@@ -600,6 +600,8 @@ typedef enum {
                                     */
   CURLE_RECURSIVE_API_CALL,      /* 93 - an api function was called from
                                     inside a callback */
+  CURLE_AUTH_ERROR,              /* 94 - an authentication function returned an
+                                    error */
   CURL_LAST /* never use! */
 } CURLcode;