logs: warn not error for maven registryUrls

lib/datasource/maven/index.js

@@ -20,7 +20,7 @@ async function getPkgReleases({ lookupName, registryUrls }) {
   const versions = [];
   const dependency = getDependencyParts(lookupName);
   if (!is.nonEmptyArray(registryUrls)) {
-    logger.error(`No repositories defined for ${dependency.display}`);
+    logger.warn(`No repositories defined for ${dependency.display}`);
     return null;
   }
   const repositories = registryUrls.map(repository =>