Return nonzero exit code if there are errors at a stop point

Author: fhahn

src/librustc_driver/driver.rs

@@ -52,12 +52,22 @@ pub fn compile_input(sess: Session,
                      addl_plugins: Option<Vec<String>>,
                      control: CompileController) {
     macro_rules! controller_entry_point{($point: ident, $make_state: expr) => ({
+        let err_count;
         {
             let state = $make_state;
+            err_count = (*state.session).err_count();
             (control.$point.callback)(state);
         }
         if control.$point.stop == Compilation::Stop {
-            return;
+            let s;
+            match err_count {
+              0 => return,
+              1 => s = "build failed due to previous error".to_string(),
+              _   => {
+                s = format!("builed failed due to {} previous errors", err_count);
+              }
+            }
+            panic!(s);
         }
     })}