TC-DA-1.2: Better error messages

Test: Added a bad der file in the cd-certs dir, saw failure on
      ToT, no failure with this fix.

src/python_testing/TC_DA_1_2.py

@@ -383,7 +383,12 @@ async def test_TC_DA_1_2(self):
             if '.der' not in filename:
                 continue
             with open(os.path.join(cd_cert_dir, filename), 'rb') as f:
-                cert = x509.load_der_x509_certificate(f.read())
+                logging.info(f'Parsing CD signing certificate file: {filename}')
+                try:
+                    cert = x509.load_der_x509_certificate(f.read())
+                except ValueError:
+                    logging.info(f'File is not a valid certificate, skipping')
+                    pass
                 pub = cert.public_key()
                 ski = x509.SubjectKeyIdentifier.from_public_key(pub).digest
                 certs[ski] = pub