build should fail when there are missing headers and ignoreFailures=false (#13)

StopActionException only causes that action to stop, and continues the build with the next action.
I've changed it to instead throw GradleException, following the lead of the Test task.

http://www.gradle.org/docs/current/javadoc/org/gradle/api/tasks/StopActionException.html
Merge pull request #15 from davidmc24/issue-13

fix "ignoreFailures=false does not fail build" (issue #13)