fix(netxlite/errorsx): map "use of closed network connection" (#510)

Part of https://github.com/ooni/probe/issues/1733 and diff has been
extracted from https://github.com/ooni/probe-cli/pull/506.

internal/netxlite/errorsx/errno.go

@@ -1,5 +1,5 @@
 // Code generated by go generate; DO NOT EDIT.
-// Generated: 2021-09-08 23:09:33.336763 +0200 CEST m=+0.192836793
+// Generated: 2021-09-27 14:20:26.411929 +0200 CEST m=+0.233208042
 
 package errorsx
 
@@ -58,6 +58,7 @@ const (
 	FailureSSLUnknownAuthority     = "ssl_unknown_authority"
 	FailureSSLInvalidCertificate   = "ssl_invalid_certificate"
 	FailureJSONParseError          = "json_parse_error"
+	FailureConnectionAlreadyClosed = "connection_already_closed"
 )
 
 // failureMap lists all failures so we can match them
@@ -101,6 +102,7 @@ var failuresMap = map[string]string{
 	"ssl_unknown_authority":          "ssl_unknown_authority",
 	"ssl_invalid_certificate":        "ssl_invalid_certificate",
 	"json_parse_error":               "json_parse_error",
+	"connection_already_closed":      "connection_already_closed",
 }
 
 // classifySyscallError converts a syscall error to the