Make build fail if assertion classes could not be generated

[jgerken]

When the generator plugin fails to create assertion classes (e.g. because of package protected class see #46) the build is not failed by the plugin. The build fails later when compiling the test sources. Since it is already clear at this point that the build will fail later, the plugin should fail the build immediately or at least provide an option to enable the fail-fast behaviour.

[joel-costigliola]

Minor but fair enough