Added xml docs.

Added GraphicsContextMissingException for use when a GraphicsContext is expected but not found.

Source/OpenTK/Graphics/GraphicsExceptions.cs

@@ -15,14 +15,32 @@ namespace OpenTK.Graphics
     /// <summary>Represents errors related to unavailable graphics parameters..</summary>
     public class GraphicsModeException : Exception
     {
+        /// <summary>Constructs a new GraphicsModeException.</summary>
         public GraphicsModeException() : base() { }
+        /// <summary>Constructs a new GraphicsModeException with the given error message.</summary>
         public GraphicsModeException(string message) : base(message) { }
     }
 
     /// <summary>Represents errors related to a GraphicsContext.</summary>
     public class GraphicsContextException : Exception
     {
+        /// <summary>Constructs a new GraphicsContextException.</summary>
         public GraphicsContextException() : base() { }
+        /// <summary>Constructs a new GraphicsContextException with the given error message..</summary>
         public GraphicsContextException(string message) : base(message) { }
     }
+
+    /// <summary>
+    /// Thrown when an operation that required GraphicsContext is performed, when no
+    /// GraphicsContext is current in the calling thread.
+    /// </summary>
+    public class GraphicsContextMissingException : GraphicsContextException
+    {
+        /// <summary>Constructs a new GraphicsContextMissingException.</summary>
+        public GraphicsContextMissingException()
+            : base(String.Format(
+                "No context is current in the calling thread (ThreadId: {0}).",
+                System.Threading.Thread.CurrentThread.ManagedThreadId))
+        { }
+    }
 }