return correct exit code

examples/fib/main.rs

@@ -0,0 +1 @@
+fn main() {}

examples/fib/specification.json

@@ -0,0 +1,12 @@
+{
+    "entrypoints": {
+        "main1": {
+            "args": [
+                "BinaryName"
+            ],
+            "environment": [
+                "Stdout"
+            ]
+        }
+    }
+}

src/main.rs

@@ -98,7 +98,7 @@ fn main() {
         };
 
         match run(&args) {
-            Ok(_) => exitcode::OK,
+            Ok(code) => code,
             Err(e) => {
                 error!("error: {}", e);
                 -1