Only warn if a referenced repository can not be loaded

Currently Tycho fails completely if one of the referenced repositories
can not be loaded, as it is hard to control references and they probably
are gone over times this makes the repository that use them unusable.

This now only warns if a referenced repository can not be loaded instead
of failing completely.

(cherry picked from commit 1f7f266)

tycho-core/src/main/java/org/eclipse/tycho/p2resolver/TargetPlatformFactoryImpl.java

@@ -406,7 +406,13 @@ private LinkedHashSet<IInstallableUnit> gatherExternalInstallableUnits(
         Set<URI> loaded = new HashSet<>();
         for (MavenRepositoryLocation location : completeRepositories) {
             artifactRepositories.add(location.getURL());
-            loadMetadataRepository(location, metadataRepositories, loaded, artifactRepositories, includeReferences);
+            try {
+                loadMetadataRepository(location, metadataRepositories, loaded, artifactRepositories, includeReferences);
+            } catch (ProvisionException e) {
+                String idMessage = location.getId() == null ? "" : " with ID '" + location.getId() + "'";
+                throw new RuntimeException(
+                        "Failed to load p2 repository" + idMessage + " from location " + location.getURL(), e);
+            }
         }
         if (includeLocalMavenRepo) {
             metadataRepositories.add(localMetadataRepository);
@@ -429,30 +435,29 @@ private LinkedHashSet<IInstallableUnit> gatherExternalInstallableUnits(
 
     private void loadMetadataRepository(MavenRepositoryLocation location,
             List<IMetadataRepository> metadataRepositories, Set<URI> loaded, Set<URI> artifactRepositories,
-            boolean includeReferences) {
+            boolean includeReferences) throws ProvisionException {
         if (loaded.add(location.getURL().normalize())) {
-            try {
-                IMetadataRepository repository = remoteMetadataRepositoryManager.loadRepository(location.getURL(),
-                        monitor);
-                metadataRepositories.add(repository);
-                if (includeReferences) {
-                    for (IRepositoryReference reference : repository.getReferences()) {
-                        if ((reference.getOptions() | IRepository.ENABLED) != 0) {
-                            if (reference.getType() == IRepository.TYPE_METADATA) {
+            IMetadataRepository repository = remoteMetadataRepositoryManager.loadRepository(location.getURL(), monitor);
+            metadataRepositories.add(repository);
+            if (includeReferences) {
+                for (IRepositoryReference reference : repository.getReferences()) {
+                    if ((reference.getOptions() | IRepository.ENABLED) != 0) {
+                        if (reference.getType() == IRepository.TYPE_METADATA) {
+                            try {
                                 loadMetadataRepository(
                                         new MavenRepositoryLocation(reference.getNickname(), reference.getLocation()),
                                         metadataRepositories, loaded, artifactRepositories, includeReferences);
-                            } else if (reference.getType() == IRepository.TYPE_ARTIFACT) {
-                                artifactRepositories.add(reference.getLocation());
+                            } catch (ProvisionException e) {
+                                logger.warn("Loading referenced repository failed: " + e.getMessage(),
+                                        logger.isDebugEnabled() ? e : null);
                             }
+                        } else if (reference.getType() == IRepository.TYPE_ARTIFACT) {
+                            artifactRepositories.add(reference.getLocation());
                         }
                     }
                 }
-            } catch (ProvisionException e) {
-                String idMessage = location.getId() == null ? "" : " with ID '" + location.getId() + "'";
-                throw new RuntimeException(
-                        "Failed to load p2 repository" + idMessage + " from location " + location.getURL(), e);
             }
+
         }
     }