added error messages when the Kieker nature could not be added;

...toring/src/kieker/tools/eclipse/monitoring/Activator.java

...ols/eclipse/monitoring/handlers/ProjectNatureHandler.java

....tools.eclipse.repository/target/repository/artifacts.jar

...ols.eclipse.repository/target/repository/artifacts.xml.xz

...er.tools.eclipse.repository/target/repository/content.jar

...tools.eclipse.repository/target/repository/content.xml.xz

...tures/kieker.tools.eclipse.feature_1.0.0.201801101605.jar

...tures/kieker.tools.eclipse.feature_1.0.0.201801101649.jar

...ieker.tools.eclipse.repository/target/repository/p2.index

...gins/kieker.tools.eclipse.analysis_1.0.0.201801101649.jar

...ns/kieker.tools.eclipse.monitoring_1.0.0.201801101649.jar