Added Exceptions.cs which contains OpenTK exceptions.

Source/OpenTK/Exceptions.cs

@@ -0,0 +1,36 @@
+#region --- License ---
+/* Copyright (c) 2006, 2007 Stefanos Apostolopoulos
+ * See license.txt for license info
+ */
+#endregion
+
+using System;
+
+namespace OpenTK
+{
+
+    #region public class ContextExistsException : ApplicationException
+
+    /// <summary>
+    /// This exception is thrown when a GLContext property cannot be changed after creation.
+    /// </summary>
+    public class ContextExistsException : ApplicationException
+    {
+        string msg;
+
+        public ContextExistsException(string message)
+        {
+            msg = message;
+        }
+
+        public override string Message
+        {
+            get
+            {
+                return msg;
+            }
+        }
+    }
+
+    #endregion
+}