Added error markers for maven-bundle-plugins with duplicated goals

Added error markers if there are multiple executions found.  Also added simplified unit test to check for error markers.
This is redundant, m2e-core already removes all lifecycle mapping error markers before calling into AbstractProjectConfigurator#configure

Also, you really want to remove error markers unconditionally, i.e. before `executions.size() > 1` check. Otherwise, the error markers won't be removed after the user fixes pom.xml configuration.