## Summary
This PR improves error related things around assignment nodes, mainly
the following:
1. Rename parse error variant:
a. `AssignmentError` -> `InvalidAssignmentTarget`
b. `NamedAssignmentError` -> `InvalidNamedAssignmentTarget`
c. `AugAssignmentError` -> `InvalidAugmnetedAssignmentTarget`
2. Add `InvalidDeleteTarget` for invalid `del` targets
a. Add helper function to check if it's a valid delete target similar to
other target check functions.
4. Fix: named assignment target can only be a `Name` node
## Test Plan
Various test cases locally. As mentioned in my previous PR, I want to
keep the testing part separate.
b65e3fb335