Prevent empty warnings in case of certificate verification failures

Fixes scrapy#1976

Author: redapple

scrapy/core/downloader/tls.py

@@ -44,7 +44,9 @@ def _identityVerifyingInfoCallback(self, connection, where, ret):
                 try:
                     verifyHostname(connection, self._hostnameASCII)
                 except VerificationError as e:
-                    logger.warning(e)
+                    logger.warning(
+                        'Remote certificate is not valid for hostname "{}"; {}'.format(
+                            self._hostnameASCII, e))
 
 except ImportError:
     # ImportError should not matter for older Twisted versions