I've attached a simple patch to add a CURLINFO_ERRORBUFFER option.

Here's why I think it's useful. When I was using the easy API it was,
well, easy to use an error buffer. I'd allocate it on the stack, set it
in the handle, make the connection, process errors if necessary, then
use curl_easy_reset(). But in transitioning to the multi API one tends
to put the handle on the multi stack at one place and time and check it
for errors/take it off/deallocate it at another place and time. So it's
no longer possible to allocate the buffer on the stack; it must be
malloc-ed. But then how to get the pointer back later to be printed or
freed?

If there's a way to solve this without CURLINFO_ERRORBUFFER, please tell me.

Thanks,
MB