Allow Api::Error to take a printf style format string so that useful error message can be constructed more easily

[a-siva]

Index: /Users/benl/svn-work/dart/dart/runtime/vm/dart_api_impl.cc
===================================================================

--- /Users/benl/svn-work/dart/dart/runtime/vm/dart_api_impl.cc (revision 1149)
+++ /Users/benl/svn-work/dart/dart/runtime/vm/dart_api_impl.cc (working copy)
@@ -504,7 +504,9 @­@
  cls_name ^= param.raw();
  const Class& cls = Class::Handle(lib.LookupClass(cls_name));
  if (cls.IsNull()) {

• return Api::Error("Specified class does not exist");
• const char *classname;
• Dart_StringToCString(name, &classname);
• return Api::Error(classname);
    }
    return Api::NewLocalHandle(cls);
   }

I really wanted to make the error more heplful, but I couldn't figure
out how, mainly because if anything was wrong with the build, I didn't
get any diagnostics, just:

[turnidge]

I agree. In fact, I had already added a TODO in dart_api.h regarding this:

DART_EXPORT Dart_Handle Dart_Error(const char* error);
// TODO(turnidge): Accept printf-style args here.

---

Added Accepted label.

[turnidge]

Added Fixed label.