fix a few bugs and improve error handling:

introduce extended error status as a start of more helpful error logging;

instead of aborting the process when we get a bucket-straddling char too
wide to handle, trace the problem and return an error;

when we get to the end of the brigade and we're still waiting for the rest
of a character, return an error instead of ignoring the problem;

if we get 0 bytes when reading from a pipe, don't fret (segfault); skip
over this empty bucket and read the next one;

don't set done if send_downstream() returns APR_SUCCESS; set it for the
opposite condition; that bug kept us from processing our entire input
brigade for brigades that caused lots of output;


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86164 13f79535-47bb-0310-9956-ffa450edef68