Added back propagation of network exceptions through endpoint calls.
Re-implemented get_conn_failed so that we can check whether or not an endpoint's partner connection has failed.
When we are propagating commit messages, we do not want to try to propagate the message to our partner if the connection with the partner has failed. Note that this will happen when we handle a NetworkException, since we will try to commit but the connection has already gone down. Notably this results in inconsistent state between endpoints; we will commit but the other endpoint will simply back out. It is left up to the programmer to handle this inconsistency in their catch block.
When we handle endpoint calls we should correctly parse the call results and raise the appropriate exceptions when needed.