Catch exceptions from parse_qs_bytes in POST bodies and log them.

This is consistent with error handling for multipart/form-data.
parse_qs is very liberal, but the conversion to unicode
on python 3 may fail.  This is most common for binary data sent from
clients where x-www-form-urlencoded is the default, which is not
actually intended to be interpreted as arguments.

Closes #921.