bpo-28182: Expose OpenSSL verification results (#3412)

The SSL module now raises SSLCertVerificationError when OpenSSL fails to
verify the peer's certificate. The exception contains more information about
the error.

Original patch by Chi Hsuan Yen

Signed-off-by: Christian Heimes <christian@python.org>
diff --git a/Lib/ssl.py b/Lib/ssl.py
index 1f3a31a..062e802 100644
--- a/Lib/ssl.py
+++ b/Lib/ssl.py
@@ -104,7 +104,7 @@
 from _ssl import _SSLContext, MemoryBIO, SSLSession
 from _ssl import (
     SSLError, SSLZeroReturnError, SSLWantReadError, SSLWantWriteError,
-    SSLSyscallError, SSLEOFError,
+    SSLSyscallError, SSLEOFError, SSLCertVerificationError
     )
 from _ssl import txt2obj as _txt2obj, nid2obj as _nid2obj
 from _ssl import RAND_status, RAND_add, RAND_bytes, RAND_pseudo_bytes