Do not use exceptions as a result pattern

[y9san9]

Now lots of methods are made in the way where you must check for exceptions in order to everything work fine. It's not a big deal though since you always try-catch your code using SDK because it can fail at any moment due to network conditions. However, using exceptions as a result doesn't give a clear contract of the method when looking at signature. Who will think when using users.edit to check whether username were occupied or not? If we make this with sealed, hopefully, this will be more clear and obvious.