failf() now only overwrites the error buffer the first time it gets called

for each *_perform(). It makes things a lot easier, as the first one that
detects the error get to write the final error reason...

lib/sendf.c

@@ -140,8 +140,10 @@ void Curl_failf(struct SessionHandle *data, const char *fmt, ...)
 {
   va_list ap;
   va_start(ap, fmt);
-  if(data->set.errorbuffer)
+  if(data->set.errorbuffer && !data->state.errorbuf) {
     vsnprintf(data->set.errorbuffer, CURL_ERROR_SIZE, fmt, ap);
+    data->state.errorbuf = TRUE; /* wrote error string */
+  }
   va_end(ap);
 }