Throw a more descriptive exception when blocking 'eval' via CSP.

src/contexts.cc

Index: src/contexts.cc
diff --git a/src/contexts.cc b/src/contexts.cc
index 169d9a12232772a523145e2b3c45db186f02bb1e..93c9795404bb52690c6cc3c4e1b560b4f9a076c6 100644
--- a/src/contexts.cc
+++ b/src/contexts.cc
@@ -305,6 +305,18 @@ void Context::ClearOptimizedFunctions() {
 }
 
 
+Handle<Object> Context::ErrorMessageForCodeGenerationFromStrings() {
+  Handle<Object> result(error_message_for_code_gen_from_strings());
+  if (result->IsUndefined()) {
+    const char* error =
+        "Code generation from strings disallowed for this context";
+    Isolate* isolate = Isolate::Current();
+    result = isolate->factory()->NewStringFromAscii(i::CStrVector(error));
+  }
+  return result;
+}
+
+
 #ifdef DEBUG
 bool Context::IsBootstrappingOrValidParentContext(
     Object* object, Context* child) {