Restore existing bindings when unbinding caught exceptions (#5256)
## Summary
In the latest release, we made some improvements to the semantic model,
but our modifications to exception-unbinding are causing some
false-positives. For example:
```py
try:
v = 3
except ImportError as v:
print(v)
else:
print(v)
```
In the latest release, we started unbinding `v` after the `except`
handler. (We used to restore the existing binding, the `v = 3`, but this
was quite complicated.) Because we don't have full branch analysis, we
can't then know that `v` is still bound in the `else` branch.
The solution here modifies `resolve_read` to skip-lookup when hitting
unbound exceptions. So when store the "unbind" for `except ImportError
as v`, we save the binding that it shadowed `v = 3`, and skip to that.
Closes #5249.
Closes #5250.
