Fail if an export command is called with multiple channels but a single-file output path

Closes #799

DiscordChatExporter.Core/Utils/PathEx.cs

@@ -17,4 +17,8 @@ public static class PathEx
 
         return buffer.ToString();
     }
+
+    public static bool IsDirectoryPath(string path) =>
+        path.EndsWith(Path.DirectorySeparatorChar) ||
+        path.EndsWith(Path.AltDirectorySeparatorChar);
 }