bindings: Throw a type error if converting an interface or callback function fails.

third_party/WebKit/Source/bindings/templates/callback_function.cpp.tmpl

 {% from 'utilities.cpp.tmpl' import v8_value_to_local_cpp_value %}
 {% filter format_blink_cpp_source_code %}
 
 {% include 'copyright_block.txt' %}
 
 #include "{{cpp_class}}.h"
 
 {% for filename in cpp_includes %}
 #include "{{filename}}"
 {% endfor %}
@@ skipping 63 matching lines @@
   }
 
   {% if return_value %}
   {{v8_value_to_local_cpp_value(return_value) | indent(2)}}
   returnValue = cppValue;
   {% endif %}
   return true;
 }
 
 {{cpp_class}}* NativeValueTraits<{{cpp_class}}>::nativeValue(v8::Isolate* isolate, v8::Local<v8::Value> value, ExceptionState& exceptionState) {
-  return {{cpp_class}}::create(ScriptState::current(isolate), value);
+  {{cpp_class}}* nativeValue = {{cpp_class}}::create(ScriptState::current(isolate), value);
+  if (!nativeValue)
+    exceptionState.throwTypeError("Unable to convert value to {{callback_function_name}}.");
+  return nativeValue;
 }
 
 }  // namespace blink
 
 {% endfilter %}{# format_blink_cpp_source_code #}