gluegen: use String.format to emit the error exception in implementation classes

Changes emitted code from:

if (__addr_ == 0) {
    throw new Exception("Method \"" + "$methodName" + "\" not available");

to:

if (__addr == 0) {
    throw new Exception(String.format("Method \"%s\" not available", "$methodName"));

This removes all the redundant error message parts for each error string and only
stores the unique method name in a string.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>