[MEDIUM] report error on buffer writes larger than buffer size

[MEDIUM] report error on buffer writes larger than buffer size

Since it's now possible to change the buffer size by configuration,
we have to take special measures against writes that are larger than
the buffer size. Before this patch, the writers would indefinitely
block, waiting for some space to free up.

With this patch, the functions simply reject the data with an
appropriate code so that the writers can either detect and process
the error or go on, but never remain blocked.

This has been tested on the stats page which does no longer hang
with buffer sizes smaller than 2.5 kB (256 bytes is even OK for
the CSV version).