Add error callbacks to all asynchronous methods

[sriharshachilakapati]

For the sake of IO, we have asynchronous methods which accept a callback for success. However we are relying on exceptions which are uncaught, and this makes it very hard for exception handling of async methods. To solve this, we need to add an error handler to these methods, and make it optional. The default error callback will just print the exception using the default logger.