Emit better error message about writable properties

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index 0282c6fb7d7e4495d902e2b0635e32b59b3a724f..ae4e2fcea6fde00fcc0142ab1deb5bf1fdfea032 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -235,8 +235,8 @@ class CallSite {
   T(SimdToNumber, "Cannot convert a SIMD value to a number")                   \
   T(UndefinedOrNullToObject, "Cannot convert undefined or null to object")     \
   T(ValueAndAccessor,                                                          \
-    "Invalid property.  A property cannot both have accessors and be "         \
-    "writable or have a value, %")                                             \
+    "Invalid property descriptor.  A property descriptor cannot both have "    \
+    "accessors and specify the value or writability of the property, %")       \

[rossberg, 2015/10/12 09:00:14]

The test seems a bit redundant now. Can we simplify this to

"Invalid property descriptor: cannot both specify accessors and a value or
writable attribute, %"

   T(VarRedeclaration, "Identifier '%' has already been declared")              \
   T(WithExpression, "% has no properties")                                     \
   T(WrongArgs, "%: Arguments list has wrong type")                             \