Add more diagnostics of autocxx autogenerated file

vsomeip-sys/build.rs

@@ -105,6 +105,11 @@ fn main() -> miette::Result<()> {
         .join("rs")
         .join("autocxx-ffi-default-gen.rs");
     println!("cargo:warning=# file_path : {}", file_path.display());
+
+    if !file_path.exists() {
+        panic!("Unable to find autocxx generated code to rewrite");
+    }
+
     if let Ok(mut contents) = fs::read_to_string(&file_path) {
         // Insert #[allow(unused_imports)] for specific lines
         contents = contents.replace(