GRAILS-6713 exit with exit code 1 if script is not found

The change contains no tests, as we were not sure how to test this properly.
One possibility is to write a test in GrailsBuildHelperTests, but these tests only use mock-classes, and not the main method in GrailsScriptRunner.
Another way of testing it is to use AbstractCliTestCase in a separate Grails-project, as we have done manually.

The test would look something like this:
````
class RunMissingScriptTests extends AbstractCliTestCase {

    void testMissingScriptShouldFailWithExitCode(){
        execute (['missingscript', '-non-interactive'])
        assert waitForProcess() == 1
    }

}
````

Contribution from Hackergarten Oslo 2014