Adapting OHIFStudyMetadataSource to use new retrieveMetadata call from ohif:study-list package.

Packages/ohif-study-list/client/lib/retrieveStudyMetadata.js

@@ -13,6 +13,11 @@ const StudyMetaDataPromises = new Map();
  * @returns {Promise} that will be resolved with the metadata or rejected with the error
  */
 OHIF.studylist.retrieveStudyMetadata = studyInstanceUid => {
+
+    // @TODO: Whenever a study metadata request has failed, its related promise will be rejected once and for all
+    // and further requests for that metadata will always fail. On failure, we probably need to remove the
+    // corresponding promise from the "StudyMetaDataPromises" map...
+
     // If the StudyMetaDataPromises cache already has a pending or resolved promise related to the
     // given studyInstanceUid, then that promise is returned
     if (StudyMetaDataPromises.has(studyInstanceUid)) {