Add optional schema compiler error messages

tools/json_schema_compiler/code.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 class Code(object):
   """A convenience object for constructing code.
 
   Logically each object should be a block of code. All methods except |Render|
   and |IsEmpty| return self.
   """
@@ skipping 31 matching lines @@
     """
     if not isinstance(obj, Code):
       raise TypeError(type(obj))
     assert self is not obj
     for line in obj._code:
       try:
         # line % () will fail if any substitution tokens are left in line
         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
 
   def Cblock(self, code):
     """Concatenates another Code object |code| onto this one followed by a
     blank line, if |code| is non-empty."""
     if not code.IsEmpty():
       self.Concat(code).Append()
     return self
@@ skipping 67 matching lines @@
     """Renders Code as a string.
     """
     return '\n'.join([l.value for l in self._code])
 
 class Line(object):
   """A line of code.
   """
   def __init__(self, value, substitute=True):
     self.value = value
     self.substitute = substitute