Show a dedicated message when mounting to an occupied Windows drive

The error message was restored from 6395f17. Fixes #2309.

src/main/java/org/cryptomator/common/mount/Mounter.java

@@ -14,6 +14,7 @@
 import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.ResourceBundle;
 
 import static org.cryptomator.integrations.mount.MountCapability.MOUNT_AS_DRIVE_LETTER;
 import static org.cryptomator.integrations.mount.MountCapability.MOUNT_TO_EXISTING_DIR;
@@ -28,13 +29,15 @@ public class Mounter {
 	private final Environment env;
 	private final WindowsDriveLetters driveLetters;
 	private final ObservableValue<ActualMountService> mountServiceObservable;
+	private final ResourceBundle resourceBundle;
 
 	@Inject
-	public Mounter(Settings settings, Environment env, WindowsDriveLetters driveLetters, ObservableValue<ActualMountService> mountServiceObservable) {
+	public Mounter(Settings settings, Environment env, WindowsDriveLetters driveLetters, ObservableValue<ActualMountService> mountServiceObservable, ResourceBundle resourceBundle) {
 		this.settings = settings;
 		this.env = env;
 		this.driveLetters = driveLetters;
 		this.mountServiceObservable = mountServiceObservable;
+		this.resourceBundle = resourceBundle;
 	}
 
 	private class SettledMounter {
@@ -97,7 +100,12 @@ private Runnable prepareMountPoint() throws IOException {
 				}
 			} else {
 				var mpIsDriveLetter = userChosenMountPoint.toString().matches("[A-Z]:\\\\");
-				if (!mpIsDriveLetter && canMountToParent && !canMountToDir) {
+				if (mpIsDriveLetter) {
+					if (driveLetters.getOccupied().contains(userChosenMountPoint)) {
+						var message = String.format(resourceBundle.getString("unlock.error.customPath.driveLetterOccupied"), userChosenMountPoint);
+						throw new IllegalMountPointException(message);
+					}
+				} else if (canMountToParent && !canMountToDir) {
 					MountWithinParentUtil.prepareParentNoMountPoint(userChosenMountPoint);
 					cleanup = () -> {
 						MountWithinParentUtil.cleanup(userChosenMountPoint);

src/main/resources/i18n/strings.properties

@@ -128,6 +128,7 @@ unlock.error.customPath.message=Unable to mount vault to custom path
 unlock.error.customPath.description.notSupported=If you wish to keep using the custom path, please go to the preferences and select a volume type that supports it. Otherwise, go to the vault options and choose a supported mount point.
 unlock.error.customPath.description.notExists=The custom mount path does not exist. Either create it in your local filesystem or change it in the vault options.
 unlock.error.customPath.description.generic=You have selected a custom mount path for this vault, but using it failed with the message: %s
+unlock.error.customPath.driveLetterOccupied=Drive letter "%s" is already in use.
 ## Hub
 hub.noKeychain.message=Unable to access device key
 hub.noKeychain.description=In order to unlock Hub vaults, a device key is required, which is secured using a keychain. To proceed, enable “%s” and select a keychain in the preferences.