Create non-existing path in setLocationAction()

When using qbittorrent-nox it is not always possible to manually create
the target path for torrent moving. This commit allows automatic path
creation. It also allows to display error messages in the
'Set location' window.

src/webui/api/torrentscontroller.cpp

@@ -737,9 +737,16 @@ void TorrentsController::setLocationAction()
     const QStringList hashes {params()["hashes"].split("|")};
     const QString newLocation {params()["location"].trimmed()};
 
-    // check if the location exists
-    if (newLocation.isEmpty() || !QDir(newLocation).exists())
-        return;
+    if (newLocation.isEmpty())
+        throw APIError(APIErrorType::BadParams, tr("Save path is empty"));
+        
+    // try to create the location if it does not exist
+    if (!QDir(newLocation).mkpath("."))
+        throw APIError(APIErrorType::Conflict, tr("Cannot make save path"));
+    
+    // check permissions
+    if (!QFileInfo(newLocation).isWritable())
+        throw APIError(APIErrorType::AccessDenied, tr("Cannot write to directory"));
 
     applyToTorrents(hashes, [newLocation](BitTorrent::TorrentHandle *torrent)
     {