MINOR: qpack: prepare error renaming

MINOR: qpack: prepare error renaming

There is two distinct enums both related to QPACK error management. The
first one is dedicated to RFC defined code. The other one is a set of
internal values returned by qpack_decode_fs(). There has been issues
discovered recently due to the confusion between them.

Rename internal values with the prefix QPACK_RET_*. The older name
QPACK_ERR_* will be used in a future commit for the first enum.