add nullable qualifier to return types that can be nil, to support Swift 2 try-catch syntax

[dkharrat]

This change adds the nullable qualifier to return types of Objective-C methods that can return a nil due to errors. This change makes these methods compatible with the Swift 2 try-catch syntax. In other words, before the change, you had to do this:

let error: NSError?
let msg = Message2(data, error: &error)
if (error != nil) {
    // handle error
} else {
    // use msg
}

After the change, you can use the try-catch syntax in Swift 2:

do {
    let msg = try Message2(data)
    // use msg 
} catch (let error as NSError) {
    // handle error
}

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok to test.

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[dkharrat]

I signed it!

[googlebot]

CLAs look good, thanks!

[thomasvl]

Can you fix the indent on these so the colons still all line up?

[dkharrat]

@thomasvl sure, I've pushed the branch with the indentation fixed.

[thomasvl]

Thanks and thanks for the patch!