Show better error message if backup directory doesn't exist

Author: Klocman

source/BulkCrapUninstaller/Forms/Windows/JunkRemoveWindow.cs

@@ -142,6 +142,11 @@ private void buttonAccept_Click(object sender, EventArgs e)
         private void CreateBackup(string backupPath)
         {
             var dir = Path.Combine(backupPath, GetUniqueBackupName());
+            if (string.IsNullOrEmpty(backupPath) || !Directory.Exists(backupPath))
+            {
+                PremadeDialogs.GenericError($"The parent directory does not exist:\n{backupPath}\n\nPlease choose a different backup location.");
+                throw new OperationCanceledException();
+            }
             try
             {
                 Directory.CreateDirectory(dir);
@@ -367,7 +372,7 @@ private void RunBackup(string targetdir)
             if (failed.Any())
             {
                 failed.Sort();
-                
+
                 // Prevent the dialog from getting too large
                 if (failed.Count > 6) failed = failed.Take(5).Concat(new[] { "... (check log for the full list)" }).ToList();