Honor coverage process exit code

``CoverageProcess`` and ``DefaultCoverageGenerator`` did not honor the ``ExitCode`` sent from the slave process. Thus the build will not fail although there is a bug in pitest/during coverage generation. This is related to pull request #126 - this fix will fail maven-verification jobs because of the ``org.junit.ClassRule`` import used in the pull request.

This is also related to #105 but is not a fix for that: The ExitCode will still be OK if there is a bad JUnit runner or bad test (e.g. a test that imports a class not available on classpath).