Add src files

src/LibNftables/NftException.cs

@@ -0,0 +1,33 @@
+using System;
+
+namespace LibNftables;
+
+/// <summary>
+/// Base exception for managed nftables failures.
+/// </summary>
+public class NftException : InvalidOperationException
+{
+    /// <summary>
+    /// Initializes a new exception instance.
+    /// </summary>
+    /// <param name="message">Error message.</param>
+    /// <param name="nativeErrorCode">Native error code, when available.</param>
+    /// <param name="nativeErrorOutput">Native error output, when available.</param>
+    /// <param name="innerException">Optional inner exception.</param>
+    public NftException(string message, int nativeErrorCode = 0, string? nativeErrorOutput = null, Exception? innerException = null)
+        : base(message, innerException)
+    {
+        NativeErrorCode = nativeErrorCode;
+        NativeErrorOutput = nativeErrorOutput;
+    }
+
+    /// <summary>
+    /// Gets the native error code.
+    /// </summary>
+    public int NativeErrorCode { get; }
+
+    /// <summary>
+    /// Gets native error output text.
+    /// </summary>
+    public string? NativeErrorOutput { get; }
+}