Raise proper NameError on del when name is not defined

Closes RustPython#929.

Author: Jongy

tests/snippets/delete.py

@@ -1,4 +1,4 @@
-from testutils import assert_raises
+from testutils import assert_raises, assertRaises
 
 a = 1
 del a
@@ -16,3 +16,6 @@ class MyObject: pass
 del (x, y)
 assert_raises(NameError, lambda: x)
 assert_raises(NameError, lambda: y)
+
+with assertRaises(NameError):
+    del y

vm/src/frame.rs

@@ -124,7 +124,7 @@ impl Scope {
 pub trait NameProtocol {
     fn load_name(&self, vm: &VirtualMachine, name: &str) -> Option<PyObjectRef>;
     fn store_name(&self, vm: &VirtualMachine, name: &str, value: PyObjectRef);
-    fn delete_name(&self, vm: &VirtualMachine, name: &str);
+    fn delete_name(&self, vm: &VirtualMachine, name: &str) -> PyResult;
     fn load_cell(&self, vm: &VirtualMachine, name: &str) -> Option<PyObjectRef>;
     fn store_cell(&self, vm: &VirtualMachine, name: &str, value: PyObjectRef);
     fn load_global(&self, vm: &VirtualMachine, name: &str) -> Option<PyObjectRef>;
@@ -169,8 +169,8 @@ impl NameProtocol for Scope {
         self.get_locals().set_item(key, value, vm).unwrap();
     }
 
-    fn delete_name(&self, vm: &VirtualMachine, key: &str) {
-        self.get_locals().del_item(key, vm).unwrap();
+    fn delete_name(&self, vm: &VirtualMachine, key: &str) -> PyResult {
+        self.get_locals().del_item(key, vm)
     }
 
     fn load_global(&self, vm: &VirtualMachine, name: &str) -> Option<PyObjectRef> {
@@ -1071,8 +1071,10 @@ impl Frame {
     }
 
     fn delete_name(&self, vm: &VirtualMachine, name: &str) -> FrameResult {
-        self.scope.delete_name(vm, name);
-        Ok(None)
+        match self.scope.delete_name(vm, name) {
+            Ok(_) => Ok(None),
+            Err(_) => Err(vm.new_name_error(format!("name '{}' is not defined", name))),
+        }
     }
 
     fn load_name(