MINOR: quic: define a generic QUIC error type

MINOR: quic: define a generic QUIC error type

Define a new structure quic_err to abstract a QUIC error type. This
allows to easily differentiate a transport and an application error
code. This simplifies error transmission from QUIC MUX and H3 layers.

This new type is defined in quic_frame module. It is used to replace
<err_code> field in <quic_conn>. QUIC_FL_CONN_APP_ALERT flag is removed
as it is now useless.

Utility functions are defined to be able to quickly instantiate
transport, tls and application errors.