FutureReturnValueIgnored analogue for CompletionStages

Hi! I have a proposal for a new check based on a situation that came up at work. Here is (essentially) our code:

```
CompletionStage c = CompletableFuture.completedFuture(null);
c.thenRun(() -> {...});
```

Error Prone compiles this with no complaints. However, if the type of `c` is changed to `CompletableFuture`, we get a warning about the return value of `thenRun` being ignored. FutureReturnValueIgnored kicks in because the return value is known to be a `CompletableFuture` rather than just a `CompletionStage`.

Since it seems equally dangerous to ignore a returned `CompletionStage` - exceptions thrown by the lambda above are suppressed - maybe Error Prone should warn about that too?