MINOR: quic: extend return value during TP parsing

MINOR: quic: extend return value during TP parsing

Extend API used for QUIC transport parameter decoding. This is done via
the introduction of a dedicated enum to report the various error
condition detected. No functional change should occur with this patch,
as the only returned code is QUIC_TP_DEC_ERR_TRUNC, which results in the
connection closure via a TLS alert.

This patch will be necessary to properly reject transport parameters
with the proper CONNECTION_CLOSE error code. As such, it should be
backported up to 2.6 with the following series.