Don't pop exeption for context

vm/src/frame.rs

@@ -739,8 +739,8 @@ impl Frame {
                 };
                 let context = match argc {
                     0 => vm.get_none(), // We have already got the exception,
-                    _ => match vm.pop_exception() {
-                        Some(exc) => exc,
+                    _ => match vm.last_exception() {
+                        Some(exc) => exc.clone(),
                         None => vm.get_none(),
                     },
                 };

vm/src/vm.rs

@@ -931,6 +931,17 @@ impl VirtualMachine {
     pub fn pop_exception(&self) -> Option<PyObjectRef> {
         self.exceptions.borrow_mut().pop()
     }
+
+    pub fn last_exception(&self) -> Option<Ref<PyObjectRef>> {
+        let exceptions = self.exceptions.borrow();
+        if exceptions.is_empty() {
+            None
+        } else {
+            Some(Ref::map(self.exceptions.borrow(), |exceptions| {
+                exceptions.last().unwrap()
+            }))
+        }
+    }
 }
 
 impl Default for VirtualMachine {