refactor(errorsx): auto-generate syscall errors mapping (#429)

* refactor(errorsx): auto-generate syscall errors mapping

Part of https://github.com/ooni/probe/issues/1505

* fix: generate independently of the platform

internal/errorsx/errno.go

@@ -0,0 +1,37 @@
+// Code generated by go generate; DO NOT EDIT.
+// Generated: 2021-07-02 15:15:17.997258 +0200 CEST m=+0.110031584
+
+package errorsx
+
+//go:generate go run ./generator/
+
+import (
+	"errors"
+	"syscall"
+)
+
+// toSyscallErr converts a syscall error to the
+// proper OONI error. Returns the OONI error string
+// on success, an empty string otherwise.
+func toSyscallErr(err error) string {
+	// filter out system errors: necessary to detect all windows errors
+	// https://github.com/ooni/probe/issues/1526 describes the problem
+	// of mapping localized windows errors.
+	var errno syscall.Errno
+	if !errors.As(err, &errno) {
+		return ""
+	}
+	switch errno {
+	case ECANCELED:
+		return FailureInterrupted
+	case ECONNREFUSED:
+		return FailureConnectionRefused
+	case ECONNRESET:
+		return FailureConnectionReset
+	case EHOSTUNREACH:
+		return FailureHostUnreachable
+	case ETIMEDOUT:
+		return FailureGenericTimeoutError
+	}
+	return ""
+}