testInvalidFormat in StatelessEngineTest causes scary warning

Running that test [1] outputs a warning like [2], which makes people think something's wrong. We might log a warning to the same channel before the test, to explain what's going on.

[1] http://svn.apache.org/repos/asf/incubator/stanbol/trunk/integration-tests/src/test/java/org/apache/stanbol/enhancer/it/StatelessEngineTest.java

[2] SEVERE: Mapped exception to response: 500 (Internal Server Error)
javax.ws.rs.WebApplicationException: com.sun.jersey.api.MessageException: A message body writer for Java class org.apache.clerezza.rdf.core.impl.SimpleMGraph, and Java type class org.apache.clerezza.rdf.core.impl.SimpleMGraph, and MIME media type application/octet-stream was not found