clean up error.h

clean up error.h

It contained quite some unused error codes. This patch gets rid of
them and thus saves some space in the resulting binaries.

More importantly, there were two instances where a source file used
an error value of another source file (which is ok), but the object
file of that other source file would not get linked into the resulting
binary (which is not ok):

	- aac_afh.c used E_AACDEC_INIT, which is not contained in
	  para_server

	- http_recv.c used E_OVERRUN which is defined only if ortp
	  support was compiled in.

The patch also fixes another bug in the ogg vorbis audio format handler
where (the positive value) E_STREAM_PAGEIN was returned instead of
-E_STREAM_PAGEIN in case ogg_stream_pagein() failed.