Detect returning nil after `err != nil` check

I would like staticcheck to detect the following:
```go
func foo() error {
  ...
  err := bar()
  if err != nil {
    return nil // BUG! Most of the time it should be `return err`
  }
  ...
}
```

Sometimes, of course, you want to explicitly return an error there, so this check will have quite a few false positives.
Most of the time, however, it will detect bugs.
