Diagnostics in SetValue() of NullValueReference and LiteralValueReference

(cherry picked from commit 31019a1)

Author: nerzhulart

Mono.Debugging/Mono.Debugging.Evaluation/LiteralValueReference.cs

@@ -108,7 +108,7 @@ public override object Value {
 				return value;
 			}
 			set {
-				throw new NotSupportedException ();
+				throw new NotSupportedException ("Assignment to a literal is not possible");
 			}
 		}
 

Mono.Debugging/Mono.Debugging.Evaluation/NullValueReference.cs

@@ -50,7 +50,7 @@ public override object Value {
 				return obj;
 			}
 			set {
-				throw new NotSupportedException ();
+				throw new NotSupportedException ("Assignment to the null literal is not possible");
 			}
 		}