public interface FailureCallback<T>
Simple interface that allows asynchronous code to perform a parameterized callback
in a similar way to GWT's callback interface. This is the base interface for
where the logic is broken down to single method interfaces so Java 8 lambdas can be used.
sender- the source of the error e.g. connection request that triggered the error, could be null
err- in case of an error triggered by an exception
errorCode- if applicable e.g. with http error codes, -1 for unknown
errorMessage- if applicable a user displayable message (can be null)