Add error information to VerifySignedData().

net/data/verify_certificate_chain_unittest/generate-target-signed-by-512bit-rsa.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 target
 certificate is signed using a weak RSA key (512-bit modulus), and so
 verification is expected to fail."""
 
 import common
 
 # Self-signed root certificate (used as trust anchor).
 root = common.create_self_signed_root_certificate('Root')
 
 # Intermediate with a very weak key size (512-bit RSA).
 intermediate = common.create_intermediate_certificate('Intermediate', root)
 intermediate.set_key(common.generate_rsa_key(512))
 
 # 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 = ['Signature verification failed']
+errors = ['RSA modulus too small',
+          'Unacceptable modulus length for RSA key',
+          'VerifySignedData failed']
 
 common.write_test_file(__doc__, chain, trusted, time, verify_result, errors)