Catch Exception instead of Throwable

Throwable has two subclasses Error and Exception and as JAVA document says:

  An Error is a subclass of Throwable that indicates serious problems that a
  reasonable application should not try to catch.

So do not catch Error here.

Bug: https://github.com/airbnb/chronos/issues/124

Signed-off-by: Chengwei Yang <yangchengwei@qiyi.com>