Fix a minor nit in the JSON schema compiler.

tools/json_schema_compiler/cpp_type_generator.py

Index: tools/json_schema_compiler/cpp_type_generator.py
===================================================================
--- tools/json_schema_compiler/cpp_type_generator.py	(revision 153519)
+++ tools/json_schema_compiler/cpp_type_generator.py	(working copy)
@@ -248,8 +248,8 @@
     """
     if ref_type in self._type_namespaces:
       return self._type_namespaces[ref_type]
-    raise KeyError(('Cannot resolve type: %s.' % ref_type) +
-        'Maybe it needs a namespace prefix if it comes from another namespace?')
+    raise KeyError('Cannot resolve type: %s. Maybe it needs a namespace prefix '
+                   'if it comes from another namespace?' % ref_type)
     return None
 
   def GetReferencedProperty(self, prop):