Replace most occurrences of os.Exit(0) in the backend and sync packages with a custom backend.Exit function that can allow "soft exit" in GUI/TUI environments to avoid killing the entire process; currently all major errors still result in killing the entire process

src/backend/exitSoft.go

@@ -0,0 +1,7 @@
+//go:build returnOnExit
+
+package backend
+
+func Exit(code int) {
+	return code
+}