Fix rust tests (#2482)

rust/tests/test_nnvm/build.rs

 extern crate ar;
 
-use std::{env, path::PathBuf, process::Command};
+use std::{
+  env,
+  fs::File,
+  path::{Path, PathBuf},
+  process::Command,
+};
 
 use ar::Builder;
-use std::fs::File;
 
 fn main() {
   let out_dir = env::var("OUT_DIR").unwrap();
@@ -11,12 +15,20 @@ fn main() {
   let output = Command::new(concat!(
     env!("CARGO_MANIFEST_DIR"),
     "/src/build_test_graph.py"
-  )).arg(&out_dir)
-    .output()
-    .expect("Failed to execute command");
-  if output.stderr.len() > 0 {
-    panic!(String::from_utf8(output.stderr).unwrap());
-  }
+  ))
+  .arg(&out_dir)
+  .output()
+  .expect("Failed to execute command");
+  assert!(
+    Path::new(&format!("{}/graph.o", out_dir)).exists(),
+    "Could not build graph lib: {}",
+    String::from_utf8(output.stderr)
+      .unwrap()
+      .trim()
+      .split("\n")
+      .last()
+      .unwrap_or("")
+  );
 
   let in_path: PathBuf = [&out_dir, "graph.o"].iter().collect();
   let out_path: PathBuf = [&out_dir, "libgraph.a"].iter().collect();