ngtcp2: Add an error code for QUIC connection errors

- Add new error code CURLE_QUIC_CONNECT_ERROR for QUIC connection
  errors.

Prior to this change CURLE_FAILED_INIT was used, but that was not
correct.

Closes https://github.com/curl/curl/pull/4754

docs/libcurl/libcurl-errors.3

@@ -259,6 +259,9 @@ An authentication function returned an error.
 .IP "CURLE_HTTP3 (95)"
 A problem was detected in the HTTP/3 layer. This is somewhat generic and can
 be one out of several problems, see the error buffer for details.
+.IP "CURLE_QUIC_CONNECT_ERROR (96)"
+QUIC connection error. This error may be caused by an SSL library error. QUIC
+is the protocol used for HTTP/3 transfers.
 .IP "CURLE_OBSOLETE*"
 These error codes will never be returned. They were used in an old libcurl
 version and are currently unused.

docs/libcurl/symbols-in-versions

@@ -152,6 +152,7 @@ CURLE_URL_MALFORMAT_USER        7.1           7.17.0
 CURLE_USE_SSL_FAILED            7.17.0
 CURLE_WEIRD_SERVER_REPLY        7.51.0
 CURLE_WRITE_ERROR               7.1
+CURLE_QUIC_CONNECT_ERROR        7.69.0
 CURLFILETYPE_DEVICE_BLOCK       7.21.0
 CURLFILETYPE_DEVICE_CHAR        7.21.0
 CURLFILETYPE_DIRECTORY          7.21.0