Suggest return raise fooException or return failwith instead of raise/failwith when inside async block

[knocte]

If a raise or failwith is placed inside an async{} block without a return before it, then the F# compiler will demand that you return a proper value (in case function returns Async<'T> where 'T is different than unit). Because of that, the developer might add by mistake code that is actually unreachable without her realizing it.

[knocte]

Mmm, unfortunately, even with a return keyword, the possible subsequent LOC after it that has another return keyword might still not be flagged as unreachable by F# compiler (TODO: file bug?). So the idea here would be that this rule detects both the absence of return keyword and/or unreachable code after it.