winutils task isAlive does not return a non-zero exit code if the requested task is not alive

Work on YARN-233 noted that winutils task isAlive does not return a non-zero exit code if the requested task is not alive. This is inconsistent with the equivalent check on Unix, kill -0, which uses a non-zero exit code to communicate status. By changing winutils task isAlive to return a non-zero exit code, we can make the error handling code on the Java side consistent, avoiding the need for additional if (WINDOWS) checks.