Add optional schema compiler error messages

tools/json_schema_compiler/code.py

Index: tools/json_schema_compiler/code.py
diff --git a/tools/json_schema_compiler/code.py b/tools/json_schema_compiler/code.py
index a0a2a5fab6dd23af92b6aa08029a44a9659d3e10..3622237a84ae30bfb8238a07ab14859739da847f 100644
--- a/tools/json_schema_compiler/code.py
+++ b/tools/json_schema_compiler/code.py
@@ -49,9 +49,9 @@ class Code(object):
         if line.substitute:
           line.value %= ()
       except TypeError:
-        raise TypeError('Unsubstituted value when concatting\n' + line)
+        raise TypeError('Unsubstituted value when concatting\n' + line.value)
       except ValueError:
-        raise ValueError('Stray % character when concatting\n' + line)
+        raise ValueError('Stray % character when concatting\n' + line.value)
       self.Append(line.value, line.substitute)
 
     return self