ui: Fix docset removing on Windows (fixes zealdocs#621)

Docsets need to be removed from registry first, to avoid rename()
failing when files are in use.

src/libs/ui/docsetsdialog.cpp

@@ -693,12 +693,20 @@ void DocsetsDialog::removeDocset(const QString &name)
     const QString tmpPath = docsetPath + QLatin1String(".deleteme.")
             + QString::number(QDateTime::currentMSecsSinceEpoch());
 
-    // Rename first to allow simultaneous installation.
-    // TODO: Check for error
-    QDir().rename(docsetPath, tmpPath);
-
     m_docsetRegistry->remove(name);
 
+    // Rename first to allow simultaneous installation.
+    if (!QDir().rename(docsetPath, tmpPath)) {
+        const QString error = tr(
+            "Cannot delete docset <b>%1</b>! Please try closing other "
+            "applications first, as they may be accessing the docset files."
+        ).arg(title);
+        QMessageBox::warning(this, QStringLiteral("Zeal"), error);
+        m_docsetsBeingDeleted.removeOne(name);
+        m_docsetRegistry->addDocset(docsetPath);
+        return;
+    }
+
     QFuture<bool> future = QtConcurrent::run([tmpPath] {
         return QDir(tmpPath).removeRecursively();
     });