[MNG-8140] Always tell why model was discarded as "invalid" (#1555)

The info is there, but we redirect user to rinse-repeat in debug mode, that that creates a TON of output.

All is there, just tell why.

---

https://issues.apache.org/jira/browse/MNG-8140

maven-core/src/main/java/org/apache/maven/internal/aether/LoggingRepositoryListener.java

@@ -96,12 +96,7 @@ public void artifactDescriptorInvalid(RepositoryEvent event) {
         buffer.append("The POM for ");
         buffer.append(event.getArtifact());
         buffer.append(" is invalid, transitive dependencies (if any) will not be available");
-
-        if (logger.isDebugEnabled()) {
-            logger.warn(buffer + ": " + event.getException().getMessage());
-        } else {
-            logger.warn(buffer + ", enable debug logging for more details");
-        }
+        logger.warn(buffer + ": " + event.getException().getMessage());
     }
 
     @Override