QUIC: moved "handshake failed" reason to send_alert.

QUIC: moved "handshake failed" reason to send_alert.

A QUIC handshake failure breaks down into several cases:
- a handshake error which leads to a send_alert call
- an error triggered by the add_handshake_data callback
- internal errors (allocation etc)

Previously, in the first case, only error code was set in the send_alert
callback.  Now the "handshake failed" reason phrase is set there as well.
In the second case, both code and reason are set by add_handshake_data.
In the last case, setting reason phrase is removed: returning NGX_ERROR
now leads to closing the connection with just INTERNAL_ERROR.

Reported by Jiuzhou Cui.