SMSApi.Api.Exception  has name the same as System.Exception. This may cost a lot of time to find what is going on

We have in our code:
```cs
try {
...
}
catch (Exception e)
{
...
}
```

It didn't work. It turned out the Exception was from SMSApi.Api namespace instead of System.

I think this is really unexpected to have library having public `Exception` class. 


