Add optional context for certificate errors.

net/data/verify_certificate_chain_unittest/generate-intermediate-lacks-signing-key-usage.py

 #!/usr/bin/python
 # Copyright (c) 2015 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.
 
 """Certificate chain with 1 intermediate and a trusted root. The intermediate
 contains a keyUsage extension, HOWEVER it does not contain the keyCertSign bit.
 Hence validation is expected to fail."""
 
 import common
 
 # Self-signed root certificate (used as trust anchor).
 root = common.create_self_signed_root_certificate('Root')
 
 # Intermediate that is missing keyCertSign.
 intermediate = common.create_intermediate_certificate('Intermediate', root)
 intermediate.get_extensions().set_property('keyUsage',
     'critical,digitalSignature,keyEncipherment')
 
 # Target certificate.
 target = common.create_end_entity_certificate('Target', intermediate)
 
 chain = [target, intermediate]
 trusted = common.TrustAnchor(root, constrained=False)
 time = common.DEFAULT_TIME
 verify_result = False
-errors = """keyCertSign bit is not set"""
+errors = """[Context] Processing Certificate
+  index: 0
+      [Error] keyCertSign bit is not set
+"""
 
 common.write_test_file(__doc__, chain, trusted, time, verify_result, errors)