Remove try/catch block on @Background/@UIThread methods

Related to issue #646 and add fix for PR #677

Because the exception is now propagated, there is no need of catching
it.

Generated code : 
```java
    @Override
    public void backgroundThrowException() {
        BackgroundExecutor.execute(new BackgroundExecutor.Task("", 0, "") {


            @Override
            public void execute() {
                ThreadActivity_.super.backgroundThrowException();
            }

        }
        );
    }
```

Notes
- The `surroundWithTryCatch` method is now unused, don't know if I have to remove it or not.