Remove "Characteristic already notifying" error

[Erhannis]

On at least some platforms, trying to notify twice on a characteristic throws an error. IMO, it should not. The user wants to be notified of changes to a characteristic - if that's already happening, nothing needs to change, no error needs to be thrown, and success is automatic. Throwing an error suggests notifications are NOT happening. Is there a counterpoint I've missed, a reason to continue throwing that error?

I see the error in Linux, and in the Web code, but if the error is present elsewhere, this issue addresses them, as well.

[rohitsangwan01]

@Erhannis, I see what you mean, and I believe this is another case of platform implementation differences. We'll work on unifying the behaviour across platforms. Thank you for pointing this out!