[BUG] stream_interface: truncate buffers when sending error messages

[BUG] stream_interface: truncate buffers when sending error messages

When an error message is returned to a client, all buffer contents
were left intact. Since the analysers were removed, the potentially
invalid data that were read had a chance to be sent too.

Now we ensure we only keep the already scheduled data in the buffer
and we truncate it after that. That means that responses with data
that must be blocked will really be blocked, and that incorrectly
chunked data will be stopped at the point where the chunking fails.