Throw illegal exception when formatting with invalid template index.

src/messages.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/api.h"
 #include "src/execution.h"
 #include "src/heap/spaces-inl.h"
 #include "src/messages.h"
@@ skipping 294 matching lines @@
   // other operations that are likely to take place (see GetLocalizedMessage
   // for example).
   return String::Flatten(result_string);
 }
 
 
 MaybeHandle<String> MessageTemplate::FormatMessage(int template_index,
                                                    Handle<String> arg0,
                                                    Handle<String> arg1,
                                                    Handle<String> arg2) {
+  Isolate* isolate = arg0->GetIsolate();
   const char* template_string;
   switch (template_index) {
 #define CASE(NAME, STRING)    \
   case k##NAME:               \
     template_string = STRING; \
     break;
     MESSAGE_TEMPLATES(CASE)
 #undef CASE
     case kLastMessage:
     default:
-      UNREACHABLE();
-      template_string = "";
-      break;
+      isolate->ThrowIllegalOperation();
+      return MaybeHandle<String>();
   }
 
-  Isolate* isolate = arg0->GetIsolate();
   IncrementalStringBuilder builder(isolate);
 
   unsigned int i = 0;
   Handle<String> args[] = {arg0, arg1, arg2};
   for (const char* c = template_string; *c != '\0'; c++) {
     if (*c == '%') {
       // %% results in verbatim %.
       if (*(c + 1) == '%') {
         c++;
         builder.AppendCharacter('%');
       } else {
         DCHECK(i < arraysize(args));
         builder.AppendString(args[i++]);
       }
     } else {
       builder.AppendCharacter(*c);
     }
   }
 
   return builder.Finish();
 }
 } }  // namespace v8::internal