refactor: remove some superfluous signals

Since now we're inheriting from Task, some signals can be reused.

launcher/tasks/Task.cpp

@@ -99,8 +99,7 @@ void Task::emitAborted()
     m_state = State::AbortedByUser;
     m_failReason = "Aborted.";
     qDebug() << "Task" << describe() << "aborted.";
-    emit failed(m_failReason);
-    emit finished();
+    emit aborted();
 }
 
 void Task::emitSucceeded()

launcher/tasks/Task.h

@@ -73,6 +73,7 @@ class Task : public QObject {
     virtual void progress(qint64 current, qint64 total);
     void finished();
     void succeeded();
+    void aborted();
     void failed(QString reason);
     void status(QString status);
 
@@ -86,7 +87,7 @@ class Task : public QObject {
    protected slots:
     virtual void emitSucceeded();
     virtual void emitAborted();
-    virtual void emitFailed(QString reason);
+    virtual void emitFailed(QString reason = "");
 
    public slots:
     void setStatus(const QString& status);