Friendly prompt that it cannot download some libraries successfully.

HMCLCore/src/main/java/org/jackhuang/hmcl/download/game/LibraryDownloadException.java

@@ -0,0 +1,34 @@
+/*
+ * Hello Minecraft! Launcher.
+ * Copyright (C) 2017  huangyuhui <huanghongxun2008@126.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see {http://www.gnu.org/licenses/}.
+ */
+package org.jackhuang.hmcl.download.game;
+
+import org.jackhuang.hmcl.game.Library;
+
+public class LibraryDownloadException extends Exception {
+    private final Library library;
+
+    public LibraryDownloadException(Library library, Throwable cause) {
+        super("Unable to download library " + library, cause);
+
+        this.library = library;
+    }
+
+    public Library getLibrary() {
+        return library;
+    }
+}

HMCLCore/src/main/java/org/jackhuang/hmcl/download/game/LibraryDownloadTask.java

@@ -55,7 +55,7 @@ public final class LibraryDownloadTask extends Task {
 
     @Override
     public Collection<? extends Task> getDependents() {
-        return library.getChecksums() != null ? Collections.singleton(xzTask) : Collections.emptySet();
+        return library.getChecksums() != null ? Collections.singleton(xzTask) : Collections.singleton(task);
     }
 
     @Override
@@ -65,20 +65,23 @@ public final class LibraryDownloadTask extends Task {
 
     @Override
     public void execute() throws Exception {
-        if (isDependentsSucceeded() && library.getChecksums() != null) {
+        if (!isDependentsSucceeded()) {
+            // Since FileDownloadTask wraps the actual exception with another IOException.
+            // We should extract it letting the error message clearer.
+            Throwable t = library.getChecksums() != null ? xzTask.getLastException() : task.getLastException();
+            if (t.getCause() != null && t.getCause() != t)
+                throw new LibraryDownloadException(library, t.getCause());
+            else
+                throw new LibraryDownloadException(library, t);
+        }
+
+        if (library.getChecksums() != null) {
             unpackLibrary(jar, FileUtils.readBytes(xzFile));
             if (!checksumValid(jar, library.getChecksums()))
                 throw new IOException("Checksum failed for " + library);
-
-            downloaded = true;
         }
     }
 
-    @Override
-    public Collection<? extends Task> getDependencies() {
-        return downloaded ? Collections.emptySet() : Collections.singleton(task);
-    }
-
     private static boolean checksumValid(File libPath, List<String> checksums) {
         try {
             if ((checksums == null) || (checksums.isEmpty())) {

HMCLCore/src/main/java/org/jackhuang/hmcl/task/Task.java

@@ -63,6 +63,16 @@ public abstract class Task {
         this.state = state;
     }
 
+    private Throwable lastException = null;
+
+    public Throwable getLastException() {
+        return lastException;
+    }
+
+    void setLastException(Throwable e) {
+        lastException = e;
+    }
+
     /**
      * The scheduler that decides how this task runs.
      */

HMCLCore/src/main/java/org/jackhuang/hmcl/task/TaskExecutor.java

@@ -190,6 +190,7 @@ public final class TaskExecutor {
         } catch (SilentException | RejectedExecutionException e) {
             // do nothing
         } catch (Exception e) {
+            task.setLastException(e);
             lastException = e;
             variables.set(LAST_EXCEPTION_ID, e);
             if (task.getSignificance().shouldLog()) {