multi: handle errors returned from socket/timer callbacks

The callbacks were partially documented to support this. Now the
behavior is documented and returning error from either of these
callbacks will effectively kill all currently ongoing transfers.

Added test 530 to verify

Reported-by: Marcelo Juchem
Fixes #8083
Closes #8089

lib/strerror.c

@@ -404,6 +404,9 @@ curl_multi_strerror(CURLMcode error)
   case CURLM_BAD_FUNCTION_ARGUMENT:
     return "A libcurl function was given a bad argument";
 
+  case CURLM_ABORTED_BY_CALLBACK:
+    return "Operation was aborted by an application callback";
+
   case CURLM_LAST:
     break;
   }