staticcheck: catch when a defer stores an error into the ether

[mvdan]

For example, https://go-review.googlesource.com/c/go/+/194638 fixes a nasty bug where ParseExprFrom would completely ignore errors under some scenarios, because the author forgot to name the result parameter err error.

This would be a tricky rule to write, but if done well, it could catch nasty hidden bugs.

Filing this issue to keep track of past examples that have been fixed, as well as false positives we should look out for.

[dominikh]

Examples of potential false positives that make this check non-trivial to implement: https://play.golang.org/p/9CYioVVhzV9